💥 Refactor add-hook! macro & change arg order

This update may potentially break your usage of add-hook! if you pass
the :local or :append properties to it. This is how they used to work:

  (add-hook! :append 'some-mode-hook #'do-something)

Thsoe properties must now follow the hooks, e.g.

  (add-hook! 'some-mode-hook :append #'do-something)

Other changes:
- Various add-hook calls have been renamed to add-hook! because I
  incorrectly assumed `defun` always returned its definition's symbol,
  when in fact, its return value is "undefined" (so sayeth the
  documentation). This should fix #1597.
- This update adds the ability to add multiple functions to hooks
  without a list:

    (add-hook! 'some-mode-hook
               #'do-something
               #'do-something-else)

- The indentation logic has been changed so that consecutive function
  symbols at indented at the same level as the first argument, but forms
  are indent like a defun.

    (add-hook! 'some-mode-hook
               #'do-something
               #'do-something-else)

    (add-hook! 'some-mode-hook
      (message "Hello"))

modules/email/mu4e/config.el

@@ -168,7 +168,7 @@
     ;; Without it, refiling (archiving), trashing, and flagging (starring) email
     ;; won't properly result in the corresponding gmail action, since the marks
     ;; are ineffectual otherwise.
-    (add-hook 'mu4e-mark-execute-pre-hook
+    (add-hook! 'mu4e-mark-execute-pre-hook
       (defun +mu4e-gmail-fix-flags-h (mark msg)
         (pcase mark
           (`trash  (mu4e-action-retag-message msg "-\\Inbox,+\\Trash,-\\Draft"))