[zk-sdk] Change ZeroBalanceProof to ZeroCiphertextProof

[samkim-crypto]

Problem

The ZeroBalanceProof certifies that a ciphertext is an encryption of the number zero. It does have applications to private transfers, but the name is unnecessarily specific to this application only. It makes sense to rename the type to something more general as proposed in #671.

Summary of Changes

Rename ZeroBalanceProof to ZeroCiphertextProof.

Fixes #

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 57.14286% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 82.2%. Comparing base (e4ec48f) to head (0c9f9b7).
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master    #1085     +/-   ##
=========================================
  Coverage    82.2%    82.2%             
=========================================
  Files         868      880     +12     
  Lines      234388   235584   +1196     
=========================================
+ Hits       192668   193689   +1021     
- Misses      41720    41895    +175     

[joncinque]

Looks good! There are also references to "zero-balance" in transcript.rs:

agave/zk-sdk/src/transcript.rs

Line 37 in 6398ddf

/// Append a domain separator for zero-balance proof.

-- will those get changed in a future PR?

[samkim-crypto]

Thanks for the review! Yes, I forgot to mention it in the PR description. There are some other things to address relating to transcript, so there will be a follow-up PR addressing those and fixing the zero-balance name there.