core: split into fmt::Show and fmt::String

fmt::Show is for debugging, and can and should be implemented for
all public types. This trait is used with `{:?}` syntax. There still
exists #[derive(Show)].

fmt::String is for types that faithfully be represented as a String.
Because of this, there is no way to derive fmt::String, all
implementations must be purposeful. It is used by the default format
syntax, `{}`.

This will break most instances of `{}`, since that now requires the type
to impl fmt::String. In most cases, replacing `{}` with `{:?}` is the
correct fix. Types that were being printed specifically for users should
receive a fmt::String implementation to fix this.

Part of #20013

[breaking-change]

src/libstd/sys/unix/fs.rs

@@ -381,7 +381,7 @@ mod tests {
                 assert_eq!(buf[2], 's' as u8);
                 assert_eq!(buf[3], 't' as u8);
             }
-            r => panic!("invalid read: {}", r),
+            r => panic!("invalid read: {:?}", r),
         }
 
         assert!(writer.read(&mut buf).is_err());