std: Rename Show/String to Debug/Display

This commit is an implementation of [RFC 565][rfc] which is a stabilization of
the `std::fmt` module and the implementations of various formatting traits.
Specifically, the following changes were performed:

[rfc]: https://github.com/rust-lang/rfcs/blob/master/text/0565-show-string-guidelines.md

* The `Show` trait is now deprecated, it was renamed to `Debug`
* The `String` trait is now deprecated, it was renamed to `Display`
* Many `Debug` and `Display` implementations were audited in accordance with the
  RFC and audited implementations now have the `#[stable]` attribute
  * Integers and floats no longer print a suffix
  * Smart pointers no longer print details that they are a smart pointer
  * Paths with `Debug` are now quoted and escape characters
* The `unwrap` methods on `Result` now require `Display` instead of `Debug`
* The `Error` trait no longer has a `detail` method and now requires that
  `Display` must be implemented. With the loss of `String`, this has moved into
  libcore.
* `impl<E: Error> FromError<E> for Box<Error>` now exists
* `derive(Show)` has been renamed to `derive(Debug)`. This is not currently
  warned about due to warnings being emitted on stage1+

While backwards compatibility is attempted to be maintained with a blanket
implementation of `Display` for the old `String` trait (and the same for
`Show`/`Debug`) this is still a breaking change due to primitives no longer
implementing `String` as well as modifications such as `unwrap` and the `Error`
trait. Most code is fairly straightforward to update with a rename or tweaks of
method calls.

[breaking-change]
Closes #21436

src/libstd/num/mod.rs

@@ -16,7 +16,7 @@
 #![stable]
 #![allow(missing_docs)]
 
-#[cfg(test)] use fmt::Show;
+#[cfg(test)] use fmt::Debug;
 use ops::{Add, Sub, Mul, Div, Rem, Neg};
 
 use marker::Copy;
@@ -322,7 +322,7 @@ pub fn test_num<T>(ten: T, two: T) where
     T: PartialEq + NumCast
      + Add<Output=T> + Sub<Output=T>
      + Mul<Output=T> + Div<Output=T>
-     + Rem<Output=T> + Show
+     + Rem<Output=T> + Debug
      + Copy
 {
     assert_eq!(ten.add(two),  cast(12i).unwrap());