Refactor out map.el usage

After some profiling, it turns out map-put and map-delete are 5-7x
slower (more on Emacs 25) than delq, setf/alist-get and add-to-list for
small lists (under 250 items), which is exactly how I've been using
them.

The only caveat is alist-get's signature is different on Emacs 25, thus
a polyfill is necessary in core-lib.

modules/feature/lookup/autoload/devdocs.el

@@ -9,7 +9,7 @@ DOCSET (a string).
 See `devdocs-alist' for the defaults. "
   (after! (:when (boundp 'devdocs-alist))
     (dolist (mode (doom-enlist modes))
-      (map-put devdocs-alist mode docset))))
+      (setf (alist-get mode devdocs-alist) docset))))
 
 ;;;###autoload
 (def-setting! :devdocs (modes docset)