Relatively straight-forward marker insertions
Make markers implement various traits
Garbage collect unused type/lifetime parameters
Phantom type parameters where no data of type T is reachable:
nonetheless, covariant seems correct.
Use Option<T> in thread-local data key: markers were failing to resolve,
presumably due to hygiene not being fully implemented.
Close up some holes in variance inference (e.g., check some missing
items, check uses in trait bounds) and have it report an error when
bivariance results. Should really add some more tests for those
missing cases!