Using glob(3) this adds simple glob support. I'm sure this isn't
sufficient for all of the pattern-matching that is possible in POSIX
shell's, but it is a good start.

This introduces a `'a Exit.t` type for handling exit codes during the
execution of a series of commands. This is useful because unlike a
`Result.t` a non-zero exit code does not always mean _unrecoverable
error_.

We create the infrastructure necessary to support shell built-ins. The
built-ins `cd` and `pwd` are added and lightly tested.