Make Either be As{Ref,Mut} over ?Sized types

[nvzqz]

The blanket impl can cover all unsized types, removing the need for an impl per specific unsized type.

[cuviper]

Please see the discussion in #28. I believe it is a breaking change to add a blanket T: ?Sized implementation -- or rather expanding the reach of the current blanket T. We've left that issue open to potentially address this in the future.

[bluss]

I'll close this since we don't have a plan for 2.0 at the moment.

Consider the different roles crates have. Crates that are used privately (local to a crate) have an easier time to go from 1.0 to 2.0. Say for example lazy_static. If you update that crate, it probably does not impact the public api of any crate.

Either is used for crate interchange in some ways, as part of the public api. Then it has ecosystem implications if we go from version 1 to 2, and crates need to update together to be compatible, some even are forced to bump from 1 to 2 themselves. At this point, I don't see evidence that we have enough to improve in either 2.0, for it to be worth the bump.