PC Refactoring 2

[95DDB]

There are a couple of pending refactoring/improvements on poly-commit that we decided to postpone:

• Avoid cloning when trimming keys. This requires passing around references and dealing with lifetimes, that turns out to be quite complex given the current code structure;
• Remove the DomainExtendedPolynomialCommitment struct and treat commitments as they were always segmented (like it was before the first PC Refactoring). This will avoid a series of issues in terms of templatization, GroupVec and GroupVecGadget