Simple snark verifier script

[s1na]

Sample script for verifying a groth16 snark proof. In this case, the circuit verifies the result of multiplication of a private input and a public one. Verifier Expects the public input to match pre_state_root, and if proof is valid, updates the state root to the multiplication result.

I also added some benchmarking (#4) which I can remove or move to a different PR if you prefer. One kind measures the elapsed time of the execution of the whole script, and the other kind adds 2 host functions which scripts can use to benchmark specific parts of their execution.