Consider removing dead code from repo

[TomAFrench]

As part of the tightening up of the interface done in #117. I noticed that we don't actually use a decent number of the functions which we've written.

• hash_to_field in blake2s.rs.
• compress_many in pedersen.rs.

We also don't use construct_public_key in schnorr.rs however that's useful for testing whereas the other two are not.

We should consider removing these functions if this package is being restricted to serve only the ACVM interface.

[TomAFrench]

We've removed so much code from here that github thinks this is a solidity project.