Backport to Emacs 25: avoid (setf (alist-get ... 'equal))

We would need to use `'equal` for comparison, but Emacs 25 only allows `'eq`.
Using `advice-add` to override `alist-get` does not work, because `setf`
has special handling for `alist-get`.

`repl.el`: Switch to a hash table which already supports multiple comparison
functions, and changing of elements even in Emacs 25.
`eshell/autoload/settings.el`: use conditional set-or-push.
Drop `doom*alist-get`, it is unused now.

Thanks to @hlissner for the reimplementation.

modules/emacs/eshell/autoload/settings.el

@@ -7,8 +7,11 @@
       (signal 'wrong-number-of-arguments (list 'even (length aliases))))
   (after! eshell
     (while aliases
-      (setf (alist-get (pop aliases) +eshell-aliases nil nil #'equal)
-            (list (pop aliases))))
+      (let ((alias (pop aliases))
+            (command (pop aliases)))
+        (if-let* ((oldval (assoc alias +eshell-aliases)))
+            (setcdr oldval (list command))
+          (push (list alias command) +eshell-aliases))))
     (when (boundp 'eshell-command-aliases-list)
       (if +eshell--default-aliases
           (setq eshell-command-aliases-list