Add support for various tapping macros

Add support for various tapping macros

A macro key can now be easily set to act as a modifier on hold, and
press a shifted key when tapped. Or to switch layers when held, and
again press a shifted key when tapped.

Various other helper defines have been created which send macros when
the key is pressed, released and tapped, cleaning up the
action_get_macro function inside keymap definitions.

The layer switching macros require a GCC extension - 'compound
statements enclosed within parentheses'. The use of this extension is
already present within the macro subsystem of this project, so its use
in this commit should not cause any additional issues.

MACRO_NONE had to be cast to a (macro_t*) to suppress compiler
warnings within some tapping macros.