- Move FWRETRACT to the planner
- Combine leveling, skew, etc. in a single modifier method
- Have kinematic and non-kinematic moves call one planner method

Co-Authored-By: ejtagle <ejtagle@hotmail.com>

Replace ARDUINO_ARCH_AVR with __AVR__ to better detect architecture for non-Arduino dev environments.  Resolves compile failure in PIO for 8-bit Teensduino targets

More info:
https://forum.pjrc.com/threads/33234-Using-Teensyduino-Selecting-Teensy-3-2-3-1-board-has-incorrect-platform-define
http://www.atmel.com/webdoc/avrlibcreferencemanual/using_tools_1using_avr_gcc_mach_opt.html