refactor(lib): simplify fn!, add-transient-hook!

* lisp/doom-lib.el:
  - (fn!): unroll the loop into a single, fast setplist that
    doesn't require a cl-lib macro (autoloading which seems to throw an
    error on flatpak/snap builds; still investigating that one).
  - (add-transient-hook!): Removes a redundant let-bind.  `sym` is
    already lexically bound outside the function. This will break
    anywhere lexical-binding is nil though. Not sure if I should cater
    to that scenario...

lisp/doom-lib.el

@@ -443,15 +443,13 @@ ARGLIST."
          (allow-other-keys arglist))
       ,@body)))
 
-(let ((i 1))
-  (dolist (sym '(%2 %3 %4 %5 %6 %7 %8 %9))
-    (put 'fn! sym (cl-incf i))))
+(setplist 'doom--fn-crawl '(%2 2 %3 3 %4 4 %5 5 %6 6 %7 7 %8 8 %9 9))
 (defun doom--fn-crawl (data args)
   (cond ((symbolp data)
          (when-let
              (pos (cond ((eq data '%*) 0)
                         ((memq data '(% %1)) 1)
-                        ((get 'fn! data))))
+                        ((get 'doom--fn-crawl data))))
            (when (and (= pos 1)
                       (aref args 1)
                       (not (eq data (aref args 1))))
@@ -730,9 +728,8 @@ advised)."
        (defun ,fn (&rest _)
          ,(format "Transient hook for %S" (doom-unquote hook-or-function))
          ,@forms
-         (let ((sym ,hook-or-function))
          (cond ((functionp sym) (advice-remove sym #',fn))
-                 ((symbolp sym)   (remove-hook sym #',fn))))
+               ((symbolp sym)   (remove-hook sym #',fn)))
          (unintern ',fn nil))
        (cond ((functionp sym)
               (advice-add ,hook-or-function ,(if append? :after :before) #',fn))