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.

tests: Expect arrays for messages instead of dicts

i18n: Add new module containing a Message class ...

This Message class will be used to format localized text later.