Separating frontend from the executor

[rachitnigam]

There is no reason for runt to tie together the frontend, which is responsible for figuring out which tests need to run, where the expect files are, etc. to the executor, which actually runs the commands in parallel and reports back results. We can use a generic intermediate form to specify which commands need to be executed for which tests:

[
  { "name": "compile:foo/one.txt", "cmd": "cc foo/one.txt", "expect": "foo/one.expect" }
]

The frontend is then responsible for computing these while the executor takes these as input and produces output results.

This will also allow us to connect to other testing frameworks like turnt.