[zk-sdk] Remove source, destination, and auditor variable names

[samkim-crypto]

Problem

As part of #671, we want to make the zk-sdk independent of spl token as much as possible. However, throughout the crate, there are variables that are referred to as the source, destination, or auditor.

Summary of Changes

Rename these variables.

• For ciphertext-commitment equality proofs, there is only one keypair, ciphertext, commitment, and openings, so I just removed the prefixes.
• For the other proofs, I thought a bit about alternative ways to rename the variables and just ended up with first, second, and third prefixes for simplicity.

Fixes #

[joncinque]

Looks great!