Semaphore e2e interface as example

[oskarth]

Problem

As an example, it'd be useful to have something that works end to end.

Acceptance criteria

A lot of the code for Semaphore already exists. This should:

• Have basic protocol / public API that can be mimicked for other constructs
• Basic Rust tests
• FFI for important functionality
• (Outside of this repo) basic integration through e.g. nwaku

[oskarth]

First two items done here https://github.com/vacp2p/zerokit/tree/master/semaphore

[oskarth]

Rest of work deprioritized assuming things go well with RLN critical path