Move ACIR checksum from using Sha to a faster checksum

[vezenovm]

Problem

The move to preprocess has reduced the proving and verification times for several circuits. Times can be referenced here (#765)

We currently take a Sha256 checksum to see whether the circuit someone is trying to prove differs from the one for which keys were generated. However, if you look at the timing in the PR linked above, for larger circuits hashing the circuit before every prove does cause some slowdown.

Solution

Move to a faster checksum mechanism such as this: https://github.com/srijs/rust-crc32fast

Alternatives considered

It would also be good to add some sort of unchecked flag. An experienced dev who is confident can avoid the check then and reduce the time of a call to nargo prove

Additional context