Convert the KEYCODE2SYSTEM and KEYCODE2CONSUMER macros to functions,
defaulting to using the macros.  The function form allows the compiler
to optimize the switch statement itself, over the macro nested
ternaries.

To enable this feature, #define USE_KEYCODE2_FUNCTION.

Testing against a random selection of avr-based keyboards, this
increased available flash by ~500 bytes. For arm-based keyboards,
the available flash increased by ~400 bytes.

As zvecr states, go bold and just commit to using the function instead
of the macro.

Verified against clang-format output.