M100 D was running too long - caused watchdog resets.
M100 I showed more free memory than a Arduino Mega has RAM.

This is an update of MarlinDev PR #196.

G20/21: support for switching input units between millimeters and
inches.
M149: support for changing input temperature units.

In support of these changes, code_value() and code_value_short() are
replaced with an array of functions which handle converting to the
proper types and/or units.

Following up on #3231

Keep "astyled" reformatting