Add support for generics in derive(Yokeable, ZeroCopyFrom)

[Manishearth]

This requires that all generics are 'static for the traits to work. It is theoretically possible to generate impls for non-'static generics, but at that point the bounds get really complicated and I don't see much of a use case for this kind of thing.

There's some tricky code involved in generating the bounds. This code isn't safety-crucial, however the 'static bounds are.

Fortunately, the safety invariants are usually handled by Yokeable::transform() proving covariance automatically, so we don't need to do much additional work except when proving covariance manually.

[nordzilla]

This all look good to me. I read through every line and what you're doing makes sense overall. I also appreciate the occasional comments that further explain what is going on. Thanks for adding tests for all of it as well.

[sffc]

Post-commit LGTM