plonk::prover: Introduce counter feature for FFTs and MSMs

[therealyingtong]

Example output from cargo test --features=counter plonk_api:

DEBUG halo2_proofs::plonk::prover: MSM_COUNTER: {33: 34}           // {msm_size: count}
DEBUG halo2_proofs::plonk::prover: FFT_COUNTER: {32: 30, 128: 31}  // {fft_size: count}

[therealyingtong]

I realise that this may not be useful for people who want a proving time estimate, but DON'T want to run create_proof.

Edit: We have now updated this PR to skip synthesis and do no-ops in best_multiexp and best_fft when #[cfg(feature) = counter]. This should significantly reduce the counting time.

Questions to explore:

• do we want to bench synthesis?
• do we want to bench multiopen and IPA?
• do we want to bench hashing into transcript?

An alternative solution would be to use std::backtrace in a no-parallel version of halo2_proofs.