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/libsyntax/parse/lexer/mod.rs

@@ -586,10 +586,10 @@ impl<'a> StringReader<'a> {
     /// `\x00` marker.
     #[inline(never)]
     fn scan_embedded_hygienic_ident(&mut self) -> ast::Ident {
-        fn bump_expecting_char<'a,D:fmt::Show>(r: &mut StringReader<'a>,
-                                               c: char,
-                                               described_c: D,
-                                               whence: &str) {
+        fn bump_expecting_char<'a,D:fmt::Debug>(r: &mut StringReader<'a>,
+                                                c: char,
+                                                described_c: D,
+                                                whence: &str) {
             match r.curr {
                 Some(r_c) if r_c == c => r.bump(),
                 Some(r_c) => panic!("expected {:?}, hit {:?}, {}", described_c, r_c, whence),