Ep14 sprint - completing change of name...

This PR completes the renaming of all files and symbols from wlv->nau; this include the file `wlvtest.py` and its occurrences in `Makefile`. Also the stray `README` file that was still in a `wlvlang` directory and all source files that were named `.wl` are now `.nl`.

There is a file called SPRINT-NOTES.txt that probably ought to be removed from this PL during merge.