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/emacs/eshell/autoload/settings.el

@@ -7,7 +7,8 @@
       (signal 'wrong-number-of-arguments (list 'even (length aliases))))
   (after! eshell
     (while aliases
-      (map-put +eshell-aliases (pop aliases) (list (pop aliases))))
+      (setf (alist-get (pop aliases) +eshell-aliases nil nil #'equal)
+            (pop aliases)))
     (when (boundp 'eshell-command-aliases-list)
       (if +eshell--default-aliases
           (setq eshell-command-aliases-list