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/thread.rs

@@ -519,14 +519,14 @@ mod test {
     fn test_unnamed_thread() {
         Thread::scoped(move|| {
             assert!(Thread::current().name().is_none());
-        }).join().map_err(|_| ()).unwrap();
+        }).join().ok().unwrap();
     }
 
     #[test]
     fn test_named_thread() {
         Builder::new().name("ada lovelace".to_string()).scoped(move|| {
             assert!(Thread::current().name().unwrap() == "ada lovelace".to_string());
-        }).join().map_err(|_| ()).unwrap();
+        }).join().ok().unwrap();
     }
 
     #[test]
@@ -662,7 +662,7 @@ mod test {
             Err(e) => {
                 type T = &'static str;
                 assert!(e.is::<T>());
-                assert_eq!(*e.downcast::<T>().unwrap(), "static string");
+                assert_eq!(*e.downcast::<T>().ok().unwrap(), "static string");
             }
             Ok(()) => panic!()
         }
@@ -676,7 +676,7 @@ mod test {
             Err(e) => {
                 type T = String;
                 assert!(e.is::<T>());
-                assert_eq!(*e.downcast::<T>().unwrap(), "owned string".to_string());
+                assert_eq!(*e.downcast::<T>().ok().unwrap(), "owned string".to_string());
             }
             Ok(()) => panic!()
         }
@@ -690,9 +690,9 @@ mod test {
             Err(e) => {
                 type T = Box<Any + Send>;
                 assert!(e.is::<T>());
-                let any = e.downcast::<T>().unwrap();
+                let any = e.downcast::<T>().ok().unwrap();
                 assert!(any.is::<u16>());
-                assert_eq!(*any.downcast::<u16>().unwrap(), 413u16);
+                assert_eq!(*any.downcast::<u16>().ok().unwrap(), 413u16);
             }
             Ok(()) => panic!()
         }