Store all editing commands in a single map

Keybinds may now refer to edit commands by name rather than referencing
functions directly. This added layer of indirection opens the door for
fully user-configurable keybinds.
Implement user-configurable keybinds

Keybinds are now loaded from a keymap file at resources/config/keymap.edn
rather than hard-coded into the editor core.
Rename load! to open!, outsource work to p/load-source