Add Cyberwizzards lookahead. Thanks a lot, Cyberwizzard.

For now only the two new files, tweaked a bit in the #ifdef
area to let them compile without LOOKAHEAD being defined.
Make lookahead basically working.

This means, modify existing code to let the lookahead algorithms
do their work. It also means to remove some unused code in
dda_lookahead.c and reordering some code to make it work with
LOOKAHEAD undefined.
Bring LOOKAHEAD into the config templates.
usb_serial.c: eliminate type punning compiler warnings
temp.c: DEBUG_USER code for raw temp readings.
Remove M84.

Teacup handles motor on/off automatically and if your
intention is to stop the printer, M0 is appropriate (and
conforming with the NIST G-code standard).

That said, M84 is kept as a synonym for M2 to enhance compatibility
with slic3rs default end-G-code.
Rename M200 to M119 to meet Sprinter & Marlin.

No functional change.