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.

core/core-lib.el

@@ -10,24 +10,7 @@
       ;; if-let and when-let are deprecated in Emacs 26+ in favor of their
       ;; if-let* variants, so we alias them for 25 users.
       (defalias 'if-let* #'if-let)
-      (defalias 'when-let* #'when-let)
-
-      ;; `alist-get' doesn't have its 5th argument before Emacs 26
-      (defun doom*alist-get (key alist &optional default remove testfn)
-        "Return the value associated with KEY in ALIST.
-If KEY is not found in ALIST, return DEFAULT.
-Use TESTFN to lookup in the alist if non-nil.  Otherwise, use `assq'.
-
-This is a generalized variable suitable for use with `setf'.
-When using it to set a value, optional argument REMOVE non-nil
-means to remove KEY from ALIST if the new value is `eql' to DEFAULT."
-        (ignore remove) ;;Silence byte-compiler.
-        (let ((x (cond ((null testfn) (assq key alist))
-                       ((eq testfn #'equal) (assoc key alist))
-                       ((cl-find key alist :key #'car :test testfn)))))
-          (if x (cdr x) default)))
-      (advice-add #'alist-get :override #'doom*alist-get))))
-
+      (defalias 'when-let* #'when-let))))
 
 ;;
 ;; Helpers