Tentative debug support (work in progress)

(Note that this is currently against the gc branch/pull request; should merge in master and re-push once that PR is merged.)

This is some rather fragile debug support, essentially just to show how it could be done, as I've GTG at this point.

The way it works here is that if you do `hit build --debug zlib`, then instead of executing commands, you will be dropped to a shell and asked to execute it. If you do `exit 0`, then the resulting artifact *will be installed*, and **should** be purged afterwards by the user. In general, just do `exit 1` to abort after debugging...

Issues remaining before I'm satisfied here:

[ ] The bash launched is not isolated enough but sets up lots and lots of variables; it'd be good if executing `set` gave just the relevant variables (`MY_DEP_DIR`), but I don't know the flags.

[ ] A debug build should not be installed. Instead, a temporary directory (such as an `_artifact` under the temporary build directory) should be made for the `$ARTIFACT` directory.

[ ] I think the syntax should be `hit debug zlib`, which a) always launches a build whether or not it is already in build store without the need to purge first, b) never installs into the final location.