Add a feature to set high priority keymaps for emulation modes

[fukamachi]

Add a new variable *emulation-mode-keymaps-functions* to set keymaps that precede other major/minor modes. This is equivalent to Emacs's emulation-mode-map-alists for emulation mode.

ref. Controlling Active Maps | Emacs manual

For the time being, vi-mode is in mind. It requires switching multiple keymaps by its "state" and has to merge with others to work better in other modes.

Talking about preceding lisp-mode's keybinds, and problems with directory-mode at #992 .

[fukamachi]

The other option to achieve the demand for vi-mode is allowing multiple keymaps from modes, something like changing mode-keymap -> mode-keymaps.
However, it's rare to need this kind of feature, so adding this to all modes is overkill and will negatively influence Lem's performance.

[cxxxr]

Hmm, I would prefer not to add more special variables.
How about separating the methods as follows?

(defmethod compute-keymaps ((global-mode emacs-mode)) ...)
(defmethod compute-keymaps ((global-mode vi-mode)) ...)

(defun all-keymaps ()
  ...
  (let ((keymaps (compute-keymaps (current-global-mode))))
    ...
  ))

This way, I feel it will be easier to expand in the future.

[fukamachi]

Thanks for your comment.
It'd be fine for me if it can return multiple keymaps that precede others.
I think emulation-mode-map-alists is Emacs's way, as Emacs Lisp doesn't have generic methods.

[cxxxr]

Thank you.
Personally, I feel this is more Common Lisp like.