Add statistics for proof gen./checking time, size (#2850)

Add statistics for proof gen./checking time, size (#2850)

This commit adds a statistic that records the total size of all proofs
generated by an instance of `SmtEngine`. The commit also moves
`SmtEngine::checkProof()` into `smt_engine.cpp` because it needs to know
the complete type of `d_stats` (and the separate file for that method
didn't seem that useful). Additionally, it changes
`smt::SmtEngine::checkProofTime` to `smt::SmtEngine::lfscCheckProofTime`
that only measures the time spent in LFSC and adds a statistic
`proof::ProofManager::proofProductionTime` that measures the proof
production time separately (also works with `get-proof`/`--dump-proof`).