main: Allow passing exe_name for better looking output

src: Move run.py to __main__.py ...

This allows running NewLang as a Python module.

src: Move all source code to src/newlang ...

The Python ecosystem works much better if we deicide our source code
is actually a module named 'newlang'.

I've been putting off changing this for a long time, but I might as
well do it now.