[WIP] bindings: implement a generic interface

[pythcoiner]

This PR introduce an abstracted interface for ffi bindings.
Approach:

• If the consumer want to initiate a coinjoin, it simply have to call initiate_coinjoin().
• If the consumer want to join a pool, there is 2 steps:
  • call list_pool() an select a pool to join
  • call join_coinjoin() w/ selected pool

[pythcoiner]

update: the rust side of interface is almost implemented, i've start bindings to C++

note: I've also added a convenience function list_coins() to interface, it's a uggly hack but useful at this point for testing purpose