srd: Wii Nunchuk: Complete rewrite, PD works now.

The Nintendo Wii Nunchuk protocol decoder, while it was in the repo for
a while, didn't really work yet (among other things, due to lack of
dumps for testing the code).

This is fixed now, the rewritten decoder works fine with most dump
files in the sigrok-dumps repo. A few minor issues remain to be
added/fixed, though.