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/toc.rs

@@ -176,7 +176,15 @@ impl TocBuilder {
     }
 }
 
+//NOTE(stage0): remove impl after snapshot
+#[cfg(stage0)]
 impl fmt::Show for Toc {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::String::fmt(self, f)
+    }
+}
+
+impl fmt::String for Toc {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         try!(write!(fmt, "<ul>"));
         for entry in self.entries.iter() {