[Deliverable] Create nim-ffi

[chair28980]

Owner: nwaku

Feature: Nim FFI

FURPS:

• F1. Provides the core logic needed to expose any synchronous or asynchronous Nim library to a C-FFI library.

• U1. Introduce new pragma definitions, such as {.ffi.}, to appropriately annotate types and procedures.

• U2. Any Nim project can use it and can be installed using Nimble,
  similarly to how nim-chronos is imported.

• U3. The interaction with the exposed C library can be done using JSON.

• U4. The interaction with the exposed C library can be done using protobuf.

• R1. Nim-FFI does not leak memory.

• R2. The exposed C library never hangs when working asynchronously.

• S1. The exposed C library can be used in Golang.

• S2. The exposed C library can be used in Rust.

• S3. The exposed C library can be used in Python.

• +1. nwaku repository uses nim-ffi to expose the existing libwaku functionality.

Checklist:

• Specs: link to specs and/or API definition
• Code: link to GitHub issues/PRs/Epic
• Dogfood: link to dogfooding session/artefact
• Docs: links to README.md or docs.waku.org (TBD)

[chair28980]

@Ivansete-status i updated the description to align with https://github.com/waku-org/pm/blob/master/FURPS/application/nim_ffi.md

I left the old version as is so you can capture anything you might need, please align/delete the old version with the updated at your convenience. Thank you!