bug#25797: Subject: 25.2; enhancement-request: expose ibuffer filter keymap as a named prefix

[ivan]

ibuffer uses "/" as a prefix key for filter commands, like
ibuffer-filter-by-name ("/ n").

As an evil user, I wanted to use a different prefix, since evil uses
"/" for searching.

I use "SPC" as a leader-key, so I decided to put the filter commands
behind "SPC /".

The ibuffer "/" prefix is an anonymous keymap, so I had to bind my new
key to the result of a lookup-key call:

(bind-map-for-mode-inherit ivan/ibuffer-leader-map ivan/leader-map
  :major-modes (ibuffer-mode)
  :bindings
  ("/" (lookup-key ibuffer-mode-map "/")))  ; <= here

This works, but it would be nice if ibuffer's "/" prefix was bound to
a named keymap, so the above line could instead be something like:

("/" ibuffer-filter-map)