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/lang/javascript/config.el

@@ -66,7 +66,7 @@
                             magic-mode-regexp-match-limit t)
          (progn (goto-char (match-beginning 1))
                 (not (sp-point-in-string-or-comment)))))
-  (map-put magic-mode-alist #'+javascript-jsx-file-p 'rjsx-mode)
+  (add-to-list 'magic-mode-alist '(+javascript-jsx-file-p . rjsx-mode))
   :config
   (set-electric! 'rjsx-mode :chars '(?\} ?\) ?. ?>))
   (add-hook! 'rjsx-mode-hook