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](https://github.com/rust-lang/rfcs/pull/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]

src/libcore/array.rs

@@ -39,7 +39,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable = "waiting for PartialEq to stabilize"]
+            #[stable]
             impl<A, B> PartialEq<[B, ..$N]> for [A, ..$N] where A: PartialEq<B> {
                 #[inline]
                 fn eq(&self, other: &[B, ..$N]) -> bool {
@@ -51,6 +51,7 @@ macro_rules! array_impls {
                 }
             }
 
+            #[stable]
             impl<'a, A, B, Rhs> PartialEq<Rhs> for [A, ..$N] where
                 A: PartialEq<B>,
                 Rhs: Deref<[B]>,
@@ -61,6 +62,7 @@ macro_rules! array_impls {
                 fn ne(&self, other: &Rhs) -> bool { PartialEq::ne(self[], &**other) }
             }
 
+            #[stable]
             impl<'a, A, B, Lhs> PartialEq<[B, ..$N]> for Lhs where
                 A: PartialEq<B>,
                 Lhs: Deref<[A]>
@@ -71,10 +73,10 @@ macro_rules! array_impls {
                 fn ne(&self, other: &[B, ..$N]) -> bool { PartialEq::ne(&**self, other[]) }
             }
 
-            #[unstable = "waiting for Eq to stabilize"]
+            #[stable]
             impl<T:Eq> Eq for [T, ..$N] { }
 
-            #[unstable = "waiting for PartialOrd to stabilize"]
+            #[stable]
             impl<T:PartialOrd> PartialOrd for [T, ..$N] {
                 #[inline]
                 fn partial_cmp(&self, other: &[T, ..$N]) -> Option<Ordering> {
@@ -98,7 +100,7 @@ macro_rules! array_impls {
                 }
             }
 
-            #[unstable = "waiting for Ord to stabilize"]
+            #[stable]
             impl<T:Ord> Ord for [T, ..$N] {
                 #[inline]
                 fn cmp(&self, other: &[T, ..$N]) -> Ordering {