Refactor STM library

[jpraynaud]

Why

We want to have more readability of the STM library and make it easier to maintain.

What

Refactor the STM library to make it easier to read and maintain.

How

• Phase 1: Re-organize STM library structure #2369
• Phase 2: Enhance STM library readability #2568
• Phase 3:
  • Add unit tests and fake values for objects
  • Share the code for creating test protocol phases artifacts (for integration test and benchmarks)
• Phase 4:
  • Error management is hard to understand
  • Many unwrap/panic that should be removed or explained (as safe unwrap)
• Phase 5:
  • Some code is hard to understand (long functions for Merkle path, ...)
• Phase 6:
  • Separate CoreVerifier functionality