Granular nix prototype

[georgeee]

Implement nix integration in a way to allow packages to be built "granularly".

I.e. not just initiating an environment and running make build, but rather let nix know about Mina codebase on per-package (per-library) level. This approach unlocks nix caching the infrequently-changed libraries and reusing these dependencies.

Branch: georgeee/granular-nix.