chore: add SchnorrSignature struct

[TomAFrench]

This helps avoid us having to merge and split the signature element arrays a couple of times, avoiding the situation where SchnorrConvert is thrown in #151. It also motivates removal of the SchnorrSlice error variant .

[phated]

These errors stemmed from us trying to construct a "signature" when we always needed a sig_s and sig_e. Instead of trying to construct & deconstruct all over the backend, I've instead just using sig_s and sig_e everywhere. The only place this had significant impact was in the Circuit -> ConstraintSystem conversion, but that uses iterators so it was fine too.

Superseded by 8c56b11 in #151