Add tweaks for signers to the non-lens tweak API

[florentc]

This adds a Cooked.Tweak.Signers module with the following tweaks:

• get signers
• modify signers
• set signers
• check predicate on signers
• check if a wallet is a signer
• check if there are signers
• add a first signer
• append new signers at the end
• append a new signer at the end
• remove signers
• remove one signer
• replace the first signer

These tweaks assume the list of signers has no duplicates but they do not ensure it.