Stabilize cmp

This patch marks `PartialEq`, `Eq`, `PartialOrd`, and `Ord` as
`#[stable]`, as well as the majorify of manual implementaitons of these
traits. The traits match the [reform
RFC](rust-lang/rfcs#439).

Along the way, two changes are made:

* The recently-added type parameters for `Ord` and `Eq` are
  removed. These were mistakenly added while adding them to `PartialOrd`
  and `PartialEq`, but they don't make sense given the laws that are
  required for (and use cases for) `Ord` and `Eq`.

* More explicit laws are added for `PartialEq` and `PartialOrd`,
  connecting them to their associated mathematical concepts.

In the future, many of the impls should be generalized; see
since generalizing later is not a breaking change.

[breaking-change]

map.rs

@@ -1261,6 +1261,7 @@ impl<K: Eq + Hash<S>, V: Clone, S, H: Hasher<S>> HashMap<K, V, H> {
     }
 }
 
+#[stable]
 impl<K: Eq + Hash<S>, V: PartialEq, S, H: Hasher<S>> PartialEq for HashMap<K, V, H> {
     fn eq(&self, other: &HashMap<K, V, H>) -> bool {
         if self.len() != other.len() { return false; }
@@ -1271,6 +1272,7 @@ impl<K: Eq + Hash<S>, V: PartialEq, S, H: Hasher<S>> PartialEq for HashMap<K, V,
     }
 }
 
+#[stable]
 impl<K: Eq + Hash<S>, V: Eq, S, H: Hasher<S>> Eq for HashMap<K, V, H> {}
 
 impl<K: Eq + Hash<S> + Show, V: Show, S, H: Hasher<S>> Show for HashMap<K, V, H> {

set.rs

@@ -572,6 +572,7 @@ impl<T: Eq + Hash<S>, S, H: Hasher<S>> HashSet<T, H> {
     }
 }
 
+#[stable]
 impl<T: Eq + Hash<S>, S, H: Hasher<S>> PartialEq for HashSet<T, H> {
     fn eq(&self, other: &HashSet<T, H>) -> bool {
         if self.len() != other.len() { return false; }
@@ -580,6 +581,7 @@ impl<T: Eq + Hash<S>, S, H: Hasher<S>> PartialEq for HashSet<T, H> {
     }
 }
 
+#[stable]
 impl<T: Eq + Hash<S>, S, H: Hasher<S>> Eq for HashSet<T, H> {}
 
 impl<T: Eq + Hash<S> + fmt::Show, S, H: Hasher<S>> fmt::Show for HashSet<T, H> {