Re-organize modules of STM library

[jpraynaud]

Why

We need to refactor the STM library in order to make it SNARK-friendly. Following the definition of the architecture in #2763, we have defined an implementation plan and we want to address the refactoring part of the plan as a first step.

What

Re-organize the modules of the STM library

How

• Create a membership_commitment module and move merkle_tree in it
• Create a signature_scheme module and move bls_multi_signature and schnorr_signature in it
• Create a proof_system module and move concatenation_proof in it
• Create a core or protocol module and move all the other modules in it