Worker wrapper package

This change defines the package that will be used by the worker wrapper
program. It implements the interface defines in the standalone interface
spec for the wrapper process.

It tests the library with a bunch of simple scripts that exercise each
of the components of the interface.

This relies on a couple mock changes in another PR which is why
the tests are failing.
yeah, I think we need an intermediate buffer for `cmd.Stdout` and `cmd.Stderr`, and the pipe it to `os.Stdout`/`os.Stderr`. In fact, looking at StdoutPipe source, Go prevents you (wisely) from using it if you've already set `cmd.Stdout`.