Use generic impls for Hash

[erickt]

cc @alexcrichton. This is my last outstanding patch from the Hash migration. It adds generic hash impls for many of the stdlib types. We might want to wait until we turn #[feature(default_type_params)]; permanently on though before we land this PR.

[erickt]

@alexcrichton: gah! It appears that GitHub no longer shows comments on an old commit after a rebase, and there's no way to see your comment. We might need to rethink how we've been doing reviews if we are going to lose all that comment history.

[alexcrichton]

It's a sad truth about using github for code review in general sadly, but perhaps I should be more liberal about commenting on the PR instead of specific commits.