- `per_sq_second` => `per_s2`
- `per_sqr_second` => `per_s2`
- `axis_steps_per_sqr_second` => `max_acceleration_steps_per_s2`

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.

need to check line 101-103

Based on MarlinFirmware/MarlinDev#200 from @eboston

As reported by https://github.com/MarlinFirmware/Marlin/pull/3182#issuecomment-222595488

This change actually does increase the binary size by about 12 bytes,
but how does it affect performance?

`FORCE_INLINE` seems to have no effect with standard optimization.

`FORCE_INLINE` seems to have no effect on code generation with standard
optimization.