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/libsyntax/test.rs

@@ -194,7 +194,7 @@ impl<'a> fold::Folder for TestHarnessGenerator<'a> {
             if !self.cx.path.is_empty() {
                 self.tested_submods.push((self.cx.path[self.cx.path.len()-1], sym));
             } else {
-                debug!("pushing nothing, sym: {}", sym);
+                debug!("pushing nothing, sym: {:?}", sym);
                 self.cx.toplevel_reexport = Some(sym);
             }
         }