[HOLD] Create a wallet library

[mcmire]

We wish to build a wallet library, which would encapsulate all shared functionality of the extension and mobile projects. This would allow us to remove the majority, if not all, of the controller initialization code that currently lives in both projects. This library would also include services, a shared JSON-RPC pipeline, etc.

This ticket is for implementing the library. It depends on completing the technical proposal in https://github.com/MetaMask/core/issues/4127. It will be updated following the results of that proposal.

See here for a prototype: https://github.com/MetaMask/core/pull/4451