Updating dependencies

[sunjay]

This PR performs "easy" updates to our dependencies. Easy updates are updates which only required a change in version number and nothing else broke when that update was performed. Someone still needs to update ena and lalrpop-intern in order for us to be completely up to date.

Some notable parts of this PR:

• Error chain version went from 0.11 to 0.12
• The FIXME introduced in Allow stable features + a note about why #128 is no longer necessary because dyn Trait has been stablized
• No need to #![allow(bare_trait_object)] anymore since we updated error chain

It's useful to update these dependencies (particularly error-chain) since now we won't get the barrage of warnings when we build chalk:

   Compiling chalk-parse v0.1.0 (file:///home/sunjay/Documents/projects/chalk/chalk-parse)                                                                                                                                                    
warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`72: chalk-parse                                                                                                                                                   
 --> chalk-parse/src/errors.rs:1:1
  |
1 | / error_chain! {
2 | | }
  | |_^ help: use the new name: `unused_doc_comments`
  |
  = note: #[warn(renamed_and_removed_lints)] on by default
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
 --> chalk-parse/src/errors.rs:1:1
  |
1 | / error_chain! {
2 | | }
  | |_^ help: use the new name: `unused_doc_comments`
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
 --> chalk-parse/src/errors.rs:1:1
  |
1 | / error_chain! {
2 | | }
  | |_^ help: use the new name: `unused_doc_comments`
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
 --> chalk-parse/src/errors.rs:1:1
  |
1 | / error_chain! {
2 | | }
  | |_^ help: use the new name: `unused_doc_comments`
  |
  = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

   Compiling chalk v0.1.0 (file:///home/sunjay/Documents/projects/chalk)                                                                                                                                                                      
warning: lint `bare_trait_object` has been renamed to `bare_trait_objects`0/72: chalk                                                                                                                                                         
 --> src/errors.rs:1:10
  |
1 | #![allow(bare_trait_object)] // FIXME -- error-chain uses old-style trait objects
  |          ^^^^^^^^^^^^^^^^^ help: use the new name: `bare_trait_objects`
  |
  = note: #[warn(renamed_and_removed_lints)] on by default

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/errors.rs:6:1
   |
6  | / error_chain! {
7  | |     links {
8  | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
9  | |     }
...  |
63 | |     }
64 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/errors.rs:6:1
   |
6  | / error_chain! {
7  | |     links {
8  | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
9  | |     }
...  |
63 | |     }
64 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/errors.rs:6:1
   |
6  | / error_chain! {
7  | |     links {
8  | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
9  | |     }
...  |
63 | |     }
64 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/errors.rs:6:1
   |
6  | / error_chain! {
7  | |     links {
8  | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
9  | |     }
...  |
63 | |     }
64 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`72: chalk                                                                                                                                                         
  --> src/bin/chalki.rs:47:1
   |
47 | / error_chain! {
48 | |     links {
49 | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
50 | |         Chalk(chalk::errors::Error, chalk::errors::ErrorKind);
...  |
56 | |     }
57 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: #[warn(renamed_and_removed_lints)] on by default
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/bin/chalki.rs:47:1
   |
47 | / error_chain! {
48 | |     links {
49 | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
50 | |         Chalk(chalk::errors::Error, chalk::errors::ErrorKind);
...  |
56 | |     }
57 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/bin/chalki.rs:47:1
   |
47 | / error_chain! {
48 | |     links {
49 | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
50 | |         Chalk(chalk::errors::Error, chalk::errors::ErrorKind);
...  |
56 | |     }
57 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)

warning: lint `unused_doc_comment` has been renamed to `unused_doc_comments`
  --> src/bin/chalki.rs:47:1
   |
47 | / error_chain! {
48 | |     links {
49 | |         Parse(chalk_parse::errors::Error, chalk_parse::errors::ErrorKind);
50 | |         Chalk(chalk::errors::Error, chalk::errors::ErrorKind);
...  |
56 | |     }
57 | | }
   | |_^ help: use the new name: `unused_doc_comments`
   |
   = note: this error originates in a macro outside of the current crate (in Nightly builds, run with -Z external-macro-backtrace for more info)