Redesign Rust types for representing wallet key material

[str4d]

The key types in our Rust crates have evolved somewhat organically over time:

• The Sapling key types were originally written for circuit usage, and extracted / extended from there.
• The Orchard key types were crafted for direct usage, but only within the Orchard context.
• The transparent key types have only been implemented as minimally as required for our usage. We don't have full transparent support like the zcashd wallet does.
• Wrapper types like RecipientAddress and UnifiedFullViewingKey were built as they were needed.

Once #578 is complete, we should look at the overall Zcash key structure, and redesign our Rust types for simplicity and cohesiveness, and to better match the actual use cases.

[str4d]

The zcash_keys crate is the direction we've headed in for addressing this issue. We still need to redesign the sapling-crypto key structures, and we still need to take input from zcashd (#578) to ensure we can represent those keys as needed.