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/librustdoc/html/layout.rs

@@ -30,7 +30,7 @@ pub struct Page<'a> {
     pub keywords: &'a str
 }
 
-pub fn render<T: fmt::Show, S: fmt::Show>(
+pub fn render<T: fmt::String, S: fmt::String>(
     dst: &mut io::Writer, layout: &Layout, page: &Page, sidebar: &S, t: &T)
     -> io::IoResult<()>
 {