std: Stabilize FromStr and parse

This commits adds an associated type to the `FromStr` trait representing an
error payload for parses which do not succeed. The previous return value,
`Option<Self>` did not allow for this form of payload. After the associated type
was added, the following attributes were applied:

* `FromStr` is now stable
* `FromStr::Err` is now stable
* `FromStr::from_str` is now stable
* `StrExt::parse` is now stable
* `FromStr for bool` is now stable
* `FromStr for $float` is now stable
* `FromStr for $integral` is now stable
* Errors returned from stable `FromStr` implementations are stable
* Errors implement `Display` and `Error` (both impl blocks being `#[stable]`)

Closes rust-lang#15138

Author: alexcrichton

reference.md

@@ -2994,7 +2994,7 @@ Some examples of call expressions:
 # fn add(x: i32, y: i32) -> i32 { 0 }
 
 let x: i32 = add(1i32, 2i32);
-let pi: Option<f32> = "3.14".parse();
+let pi: Option<f32> = "3.14".parse().ok();
 ```
 
 ### Lambda expressions