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/completion/helm/config.el

@@ -28,7 +28,7 @@
   :config
   (helm-mode +1)
   ;; helm is too heavy for find-file-at-point
-  (map-put helm-completing-read-handlers-alist 'find-file-at-point nil))
+  (add-to-list 'helm-completing-read-handlers-alist (cons #'find-file-at-point nil)))
 
 
 (def-package! helm