With the comma syntax. E.g.
(let ((maps '(some-mode-map another-mode-map)))
(map! :map ,maps))
This technique was used in the recent rewrite of the Racket
module (1baebda), but I forgot to include this change, so Racket users
have probably noticed some missing keybinds! This fixes that too.
Amend: 1baebdafb3
Ref: #7543