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]

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