`Eq` instances for `Static` and `Closure`.

`Static` and `Closure` are morally *pointers* to static values and
closures, respectively. As such, even in cases where it's non-sensical
to have `Eq` instances for the values they point to, it still make sense
to compare pointers (or "references") to them. Just as e.g. `Ptr` and
`StablePtr` have `Eq` instances, arguably so should `Static` and
`Closure`.

cc @yoeight.