* make `ElGamalKeypair` fields private
* update the rest of `zk-token-sdk` for the visibility update
* update `zk-token-proof-tests` for the visibility update
* update `zk-keygen` for the visibility update
* update `zk-token-proof` benches for the updated visibility
* cargo fmt
* rename `ElGamalKeypair::new` to `ElGamalKeypair::new_for_tests`
* add bench test for `VerifyPubkeyValidity`
* add bench test for `VerifyRangeProofU64`
* add bench test for `VerifyWithdraw`
* add bench test for `VerifyZeroBalance`
* add bench test for `VerifyGroupedCiphertextValidity`
* add bench test for `VerifyCiphertextCommitmentEquality`
* add bench test for `VerifyCiphertextCiphertextEquality`
* add bench test for `VerifyBatchedGroupedCiphertextValidity`
* add bench test for `VerifyBatchedRangeProofu64`, `VerifyBatchedRangeProofU128`, `VerifyBatchedRangeProofU256`
* add bench test for `VerifyTransfer` and `VerifyTransferWithFee`
* use add `criterion` to workspace cargo
* add bench to ci