Rework optional dependencies to discourage direct use as Cargo features

[KodrAus]

This affects #588: We've renamed the zerocopy-unstable feature to simply zerocopy. You still need the uuid_unstable RUSTFLAGS cfg to use it, but this avoids starting to accumulate a lot of dead features as they stabilize. That way if we promote zerocopy to a stable feature users don't need to change what features they enable, they can simply stop setting uuid_unstable.

This also renames all of our internal optional dependencies so that they discourage direct use by end-users. Otherwise we'd be stuck in a place where changes to what optional dependencies we use could break any dependents that set those features. Some optional dependencies are pure implementation details that end users shouldn't be tinkering with directly, but some of them aren't. So this tries to distinguish them better.