Simplify code 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

Simplify the code of the STM library.

How

• Move errors to their belonging modules
• Remove the future_proof_system feature and replace it with the existing future_snark feature
• Limit usage of <D: Digest> generics to merkle_tree module only (to be rechallenged)
• Remove BasicVerifier implementation (and create a ticket for supporting this feature in a cleaner way in the future?) (to be rechallenged)