WIP: Add ScriptInterpreter for script evaluation

This PR adds the ScriptInterpreter class, which allows to execute bitcoin Scripts.
It adds tests using data from bitcoind, to be sure it's working correctly.

Big thanks to @ryanxcharles, many parts of this code were ported from his version at https://github.com/ryanxcharles/fullnode
(nit) Let's change this to `privkey instanceof PrivateKey`. In order to avoid circular references, we may make the import in the same function, at the cost of loss of performance, but that may not even be necessary. Same for line 53