Add python superenv wrapper

This adds a simple python wrapper to superenv. The user's original PATH is stored in a new environment variable, which is referenced by the various python-related wrappers, to let python be called by formulae without having to add the user's entire `PATH` or an entire directory to the build's `PATH`.

I was also considering promoting `Formula#python` back to a supported method, which would do nothing more than figure out which python was chosen and return `"python"` or `"python3"` to simplify calling either wrapper.