Allow keymaps to assign functions to keymap codes

[dfnr2]

In the current implementation, each key code is hard-mapped to a function call or a value.

Consider an alternate approach (which eats more RAM) of maintaining a table of function calls.

Considerations: 1) might save RAM by using a hybrid approach. Self-insert values below the "action threshold", and look up function calls for values above the threshold.

2) need separate function tables for press and release.

[dfnr2]

Uses more space, but cleans up function-specific code in dispatch routines.

[dfnr2]

Explicit assignment of separate functions for press and release in the keymap has some problems:

• generates clutter in the keymap definitions
• error prone
• takes up much more space than keycodes

[dfnr2]

This functionality will be addressed with issue #26