* [New Feature] I2C position encoder support

I plan to continue improving/cleaning this up, as there areas that need work.

* let the cleanups begin.

* progress

* more progress

* comments, rename files, etc.

* clean

* Cleanups per thinkyhead

* a few more cleanups

* cleanups, bugfixes, etc.

* remove unnecessary passes_test(), additional cleanups/optimizations

* cleanups

* misc.

* Fix up I2CPEM.init() and a few other things.

* organize, fix, rename, etc.

* more optimization

* a few more tweaks

* UBL G29 -P3.1 mesh fill with distance-weighted least squares fit.

* Back to original -O0 on G29 for now.

Rather than include twice, which is tricky but may confuse

This is the easiest way to make button pin testing consistent without
renaming all the button pins. Just make a macro especially for testing
if button pins are set, since they are named consistently in the pins
files.

Following up on #3231