Contract tests

So, didn't add much in terms of tests, but I modularized the wallet code a bit, which is I believe where most of the changes come from.

Now, the functions to add inputs, outputs, and signatures are all separate, so you can be pretty flexible when assembling random transactions.

I'm not sure how to move forward with the testing..., so I'm going to start working on the cli for both hosts and clients. Idk what the design is going to be but it's just a reference client. Hopefully will have all the functions that we want.

I think that we're going to have to make a reference set of blocks that everyone can use to test. This is where having a standard way to hash the state would make sense. If we have 100 or 200 blocks that intentionally probe a bunch of problems, then you just feed it to your black box and see if the state hash is correct at the end.