Clean up key-code to function-call mappings.

[dfnr2]

Proposed architecture:

• Each keymap uses a keycode table which stores functions to call on press and release for the keycode
• Default keycode table encoded in the keycode scanner routine
• Alternate keycode table may be specified by a keymap, if desired.
• hooks mechanism already has a way to map a keycode to a function, so use the hooks mechanism to add dynamically map functions to key presses/releases when configuring keymaps.

proposed api:

include "asdf.h"

• default_keycode_table() - select default keycode map
• use_keycode_table(*table) - specify the keycode table

  include "asdf_hools.h"

• asdf_hook_assign(hook_id, *function()) - assigns function to keypress hook (e.g., KEY1_PRESS, KEY1_RELEASE, etc.)

hook functions would only need to be assigned for non-null actions. A NO-OP function is assigned by default during by hook_init().

the keyscan code would map the first user code ACTION_USER_START to hook ASDF_HOOK_USER_START, where:

• ACTION_USER_START == ACTION_USER1
• ASDF_HOOK_USER_START == ASDF_HOOK_USER1_PRESS

==> for press: hook index = ASDF_HOOK_USER_START + (code - ACTION_USER_START) ==> for release: hook index = ASDF_HOOK_USER_START + (code - ACTION_USER_START) + 1