rustdoc: incorporate stability index throughout

This commit hooks rustdoc into the stability index infrastructure in two
ways:

1. It looks up stability levels via the index, rather than by manual
attributes.

2. It adds stability level information throughout rustdoc output, rather
than just at the top header. In particular, a stability color (with
mouseover text) appears next to essentially every item that appears
in rustdoc's HTML output.

Along the way, the stability index code has been lightly refactored.

src/librustdoc/html/format.rs

@@ -37,6 +37,10 @@ pub struct FnStyleSpace(pub ast::FnStyle);
 pub struct Method<'a>(pub &'a clean::SelfTy, pub &'a clean::FnDecl);
 /// Similar to VisSpace, but used for mutability
 pub struct MutableSpace(pub clean::Mutability);
+/// Wrapper struct for properly emitting the stability level.
+pub struct Stability<'a>(pub &'a Option<clean::Stability>);
+/// Wrapper struct for emitting the stability level concisely.
+pub struct ConciseStability<'a>(pub &'a Option<clean::Stability>);
 
 impl VisSpace {
     pub fn get(&self) -> Option<ast::Visibility> {
@@ -596,3 +600,34 @@ impl fmt::Show for MutableSpace {
         }
     }
 }
+
+impl<'a> fmt::Show for Stability<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let Stability(stab) = *self;
+        match *stab {
+            Some(ref stability) => {
+                write!(f, "<a class='stability {lvl}' title='{reason}'>{lvl}</a>",
+                       lvl = stability.level.to_str(),
+                       reason = stability.text)
+            }
+            None => Ok(())
+        }
+    }
+}
+
+impl<'a> fmt::Show for ConciseStability<'a> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let ConciseStability(stab) = *self;
+        match *stab {
+            Some(ref stability) => {
+                write!(f, "<a class='stability {lvl}' title='{lvl}{colon}{reason}'></a>",
+                       lvl = stability.level.to_str(),
+                       colon = if stability.text.len() > 0 { ": " } else { "" },
+                       reason = stability.text)
+            }
+            None => {
+                write!(f, "<a class='stability Unmarked' title='No stability level'></a>")
+            }
+        }
+    }
+}