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/libregex/lib.rs

@@ -13,7 +13,6 @@
 //! Regular expressions implemented in Rust
 //!
 //! For official documentation, see the rust-lang/regex crate
-
 #![crate_name = "regex"]
 #![crate_type = "rlib"]
 #![crate_type = "dylib"]