Avoid recomputation of the credential public key during verification

[nategraf]

A did a round of profiling and optimization on the benchmark. I found that recomputaion of public key (i.e. public parameters) took about 10% of the runtime for handle_submit after sigma-rs/sigma-proofs#98

• Add a set_keypair function to avoid recomputation of the public key.
• Fix a build error and some clippy warnings.