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).

In the future, many of the impls should be generalized; see #20063.
However, there is no problem stabilizing the less general impls, since
generalizing later is not a breaking change.

r? @alexcrichton 
We may also want to explicitly spell out that this is *not* for a [partial equivalence relation](http://en.wikipedia.org/wiki/Partial_equivalence_relation) as I don't think that floats respect the mathematical notion of this. That does somewhat cast doubt to the name of the trait in my mind, but it's not necessarily the end of the world so long as the documentation is clear.