Add support for a Bitcoin client as an Etheno client

[mike-myers-tob]

It seems like it would be feasible to use a subset of Etheno's functionality with Bitcoin clients, but I wonder what the steps toward achieving that would be?

A Bitcoin client, like an Ethereum one, has a JSON-RPC interface for receiving commands, including commands to submit transactions embedded with scripts (in an "assembly"/bytecode form only). Like Ethereum, a Bitcoin blockchain begins with a "genesis block." Bitcoin doesn't have the concept of "gas," but the rest seems similar enough to pose this feature request.

Given that the analysis tools (Echidna, Manticore) can't do anything with Bitcoin Script yet, the first goal seems to be using Etheno's JSON-RPC multiplexing for differential testing of Bitcoin clients.

Notional CLI use:
etheno --bitcoin-core --bitcoin-core-args blabla

The next desirable feature for testing Bitcoin clients might be to extend Echidna to be able to mutate a Bitcoin script.