* tmk_core/common/report.h: define `enum mouse_buttons` in terms of `#define MOUSE_BTN_MASK()`
* tmk_core/common/action.c: collapse multiple `case KC_MS_BTN[1-8]:` into single `MOUSE_BTN_MASK(action.key.code - KC_MS_BTN1)`
We all love tapping on our keyboards but this is taking the piss.
This saves ~134 bytes on my ATmega32.