zcash_primitives: Refactor the transaction builder to enable applying proofs and signatures separately

[str4d]

This will result in the same kind of top-level builder API for transactions as we already have for Sapling and Orchard bundles individually:

• Construct a Builder.
• Add spends and outputs.
• Builder::build produces an unauthorized transaction.
• Methods on that unauthorized transaction construct proofs or apply signatures.
• A final method produces the authorized transaction once all bundles within it have been authorized.