Environment/Proto: replace Bls_signature and Bls12_381 with simply Bls

Context

This MR contains some cleanup of the environments (opening the structs) and in V7, both Bls_signature (used by TORU) and Bls12_381 (used by the Michelson primitive instructions) are replaced by a unifying module Bls.

This simplifies the code and is a prerequisite for tz4 accounts (!5444 (merged)). The next MR in this features set is !6302 (merged).

This MR depends on !6282 (merged) and !6291 (closed).

Manually testing the MR

This MR is a refactoring, the CI should prevail.