zcash_primitives: Refactor `Transaction` to permit omitting protocol-specific bundles

[str4d]

Draft because there are still some things that need doing:

• [x] I want some feedback on the overall approach to check the direction makes sense.
• [ ] I haven't documented much yet (spent my time getting the refactor structured correctly so the commits make sense).
• [ ] I haven't checked all the various feature flag combinations yet.
• [ ] The PR needs tests that exercise the various NoProtocol combinations.
• [x] The Authorization-ish traits are still tied to the protocol-specific crates. I think I need to introduce intermediate traits with blanket impls.
• [x] Factor out Sprout for completeness (even though we don't depend on anything for Sprout that we don't inherently need for basic Zcash support).
• [x] Factor out ZFuture.
• [ ] Needs changelog updates.