Track the source span for all nodes

[danheuck]

This updates the parser to keep track of the source span for each node as it is parsed, as suggested in #14. In nightly, this uses Span::join in nightly, while trying to provide a sensible "close enough" span in stable.

Example errors generated from these spans in nightly:

error: Invalid element
  --> test-app\src\main.rs:11:13
   |
11 |             <div/>
   |             ^^^^^^

error: Invalid element
  --> test-app\src\main.rs:14:13
   |
14 | /             <div>
15 | |                 "child"
16 | |             </div>
   | |__________________^

error: Invalid comment
  --> test-app\src\main.rs:19:13
   |
19 |             <!--"comment"-->
   |             ^^^^^^^^^^^^^^^^

error: Invalid doctype
  --> test-app\src\main.rs:22:13
   |
22 |             <!DOCTYPE rsx>
   |             ^^^^^^^^^^^^^^

error: Invalid fragment
  --> test-app\src\main.rs:25:13
   |
25 | /             <>
26 | |                 {stuff}
27 | |             </>
   | |_______________^

error: Invalid block
  --> test-app\src\main.rs:30:13
   |
30 | /             {
31 | |                 code
32 | |             }
   | |_____________^

error: Invalid block
  --> test-app\src\main.rs:35:13
   |
35 |             {inline}
   |             ^^^^^^^^

error: Invalid text
  --> test-app\src\main.rs:38:13
   |
38 |             "text"
   |             ^^^^^^

error: Invalid attribute
  --> test-app\src\main.rs:42:18
   |
42 |             <div key="value"/>
   |                  ^^^^^^^^^^^

and in stable:

error: Invalid element
  --> test-app\src\main.rs:11:14
   |
11 |             <div/>
   |              ^^^

error: Invalid element
  --> test-app\src\main.rs:14:14
   |
14 |             <div>
   |              ^^^

error: Invalid comment
  --> test-app\src\main.rs:19:17
   |
19 |             <!--"comment"-->
   |                 ^^^^^^^^^

error: Invalid doctype
  --> test-app\src\main.rs:22:23
   |
22 |             <!DOCTYPE rsx>
   |                       ^^^

error: Invalid fragment
  --> test-app\src\main.rs:25:13
   |
25 |             <>
   |             ^

error: Invalid block
  --> test-app\src\main.rs:30:13
   |
30 | /             {
31 | |                 code
32 | |             }
   | |_____________^

error: Invalid block
  --> test-app\src\main.rs:35:13
   |
35 |             {inline}
   |             ^^^^^^^^

error: Invalid text
  --> test-app\src\main.rs:38:13
   |
38 |             "text"
   |             ^^^^^^

error: Invalid attribute
  --> test-app\src\main.rs:42:18
   |
42 |             <div key="value"/>
   |                  ^^^

Resolves #14

[stoically]

Great addition, thanks for working on it!