Hackatom: Finalize integration test and refactor

[ethanfrey]

Building on #1 and #2 we can produce a clean integration test that handles both init and send, and provides some good abstractions on top of all accesses to the instance. Spend time pulling these helper methods into their own module and making the vm access in the integration test clean. This is the same code we will need to call into the vm in production - it should be reusable (outside the test itself).