Semaphore: module refactoring

[s1fr0]

Problem

Currently, zerokit semaphore module implements wrappers around API exposed by semaphore-rs, a submodule of the semaphore crate.

This doesn't allow the same flexibility and easy features extension currently allowed e.g., by the zerokit RLN module.

Furthermore, using submodules makes hard to deal with any eventually breaking change introduced by submodule's dependencies, as happened in #79. This results in a failing zerokit compilation and leaves few possibilities to solve such issue quickly without relying on forks and/or dependencies' developers response.

Acceptance criteria

• Refactor semaphore module similarly as the RLN module (public API, FFI, zk resources, MT implementation, etc.)