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/file-templates/autoload.el

@@ -2,7 +2,9 @@
 
 (defun +file-templates--set (pred plist)
   (if (null (car-safe plist))
-      (setq +file-templates-alist (map-delete +file-templates-alist pred))
+      (setq +file-templates-alist
+            (delq (assoc pred +file-templates-alist)
+                  +file-templates-alist))
     (push `(,pred ,@plist) +file-templates-alist)))
 
 ;;;###autodef