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/erlang/config.el

@@ -5,7 +5,7 @@
                   "/rebar\\.config\\(?:\\.script\\)?$"
                   ;; erlang configs
                   "/\\(?:app\\|sys\\)\\.config$"))
-  (map-put auto-mode-alist regexp 'erlang-mode))
+  (add-to-list 'auto-mode-alist (cons regexp 'erlang-mode)))
 
 
 (def-package! flycheck-rebar3