zcash_proofs: Extract separate provers from SaplingProvingContext

[str4d]

Currently, we have a single SaplingProvingContext which we initialize, use to create each Spend and Output proof (along with extra fields for the Spend description), and then call binding_sig to produce bindingSig. The reason for this workflow was that there is a data dependency on the entire transaction to create it. However, this data dependency is only a blocker on creating bindingSig due to computing bsk; the remaining components can be created independently of the whole transaction.

We should extract the per-proof creation logic from SaplingProvingContext, so they can be run independently.

[daira]

This was indeed closed by #1001, because #741 can be completed by #1023.