std: Stabilize the prelude module

This commit is an implementation of [RFC 503][rfc] which is a stabilization
story for the prelude. Most of the RFC was directly applied, removing reexports.
Some reexports are kept around, however:

* `range` remains until range syntax has landed to reduce churn.
* `Path` and `GenericPath` remain until path reform lands. This is done to
  prevent many imports of `GenericPath` which will soon be removed.
* All `io` traits remain until I/O reform lands so imports can be rewritten all
  at once to `std::io::prelude::*`.

This is a breaking change because many prelude reexports have been removed, and
the RFC can be consulted for the exact list of removed reexports, as well as to
find the locations of where to import them.

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0503-prelude-stabilization.md
[breaking-change]

Closes rust-lang#20068

reference.md

@@ -3174,6 +3174,7 @@ then the expression completes.
 Some examples of call expressions:
 
 ```
+# use std::str::from_str;
 # fn add(x: int, y: int) -> int { 0 }
 
 let x: int = add(1, 2);