Add action_tapping.[ch] for refactoring
Merge branch 'action_tapping'
Refine README.md
Fix BOOTLOADER_SIZE setting
Remove ACT_KEYMAP and ACT_OVERLAY

- Remove ACT_OVERLAY
- Rename ACT_KEYMAP to ACT_LAYER
- Add ACT_LAYER_BITOP
Refine ACT_LAYER and ACT_LAYER_TAP

- Remove ACT_LAYER_BITOP
Rename file layer_switch to action_layer
Fix ACTION_LAYER macros
Fix doc/keymap.md
Add file action_code.h
Fix ACT_MODS action and switch_default_layer command
Fix keymap of gh60
Fix doc/keymap.md for new actions
Fix keymap of hhkb for new actions
Merge branch 'action_refine'
Fix keymap for new actions