Customizable keybinding

[xiaq]

The keybinding is now fixed, but should really be customizable. New builtins needs to be introduced to modify the keybinding.

[xiaq]

An obvious solution:

• Introduce an edit: namespace.

• Expose editor builtins as functions in edit:.

• Make edit.Key a value type and provide a constructor edit:key for it.

• Introduce a table $edit:binding whose structure maps to current edit.keyBindings. For instance, with the default binding it looks like:

  [&command [&(edit:key i) $edit:fn-start-insert ...]
   &insert [&(edit:key up) $edit:fn-move-dot-up ...]
   ...]
  

• Maybe introduce a edit:bind function to help manipulate the binding table. Its interface can mimic bind of bash or bindkey of zsh.

We should be able to manipulate tables first (#11).

[xiaq]

The bind builtin now supports manipulating the keybinding table. However, it is not yet possible to bind keys to shell functions.