Derive Eq for std::cmp::Ordering, instead of using manual impl.
This allows consts of type Ordering to be used in patterns, and with feature(adt_const_params) allows using `Ordering` as a const generic parameter.
Currently, `std::cmp::Ordering` implements `Eq` using a manually written `impl Eq for Ordering {}`, instead of `derive(Eq)`. This means that it does not implement `StructuralEq`.
This commit removes the manually written impl, and adds `derive(Eq)` to `Ordering`, so that it will implement `StructuralEq`.
4ead6d9dc7