💥 revise advice naming convention (1/2)

This is first of three big naming convention updates that have been a
long time coming. With 2.1 on the horizon, all the breaking updates will
batched together in preparation for the long haul.

In this commit, we do away with the asterix to communicate that a
function is an advice function, and we replace it with the '-a' suffix.
e.g.

  doom*shut-up -> doom-shut-up-a
  doom*recenter -> doom-recenter-a
  +evil*static-reindent -> +evil--static-reindent-a

The rationale behind this change is:

1. Elisp's own formatting/indenting tools would occasionally struggle
   with | and * (particularly pp and cl-prettyprint). They have no
   problem with / and :, fortunately.
2. External syntax highlighters (like pygmentize, discord markdown or
   github markdown) struggle with it, sometimes refusing to highlight
   code beyond these symbols.
3. * and | are less expressive than - and -- in communicating the
   intended visibility, versatility and stability of a function.
4. It complicated the regexps we must use to search for them.
5. They were arbitrary and over-complicated to begin with, decided
   on haphazardly way back when Doom was simply "my private config".

Anyhow, like how predicate functions have the -p suffix, we'll adopt the
-a suffix for advice functions, -h for hook functions and -fn for
variable functions.

Other noteable changes:
- Replaces advice-{add,remove}! macro with new def-advice!
  macro. The old pair weren't as useful. The new def-advice! saves on a
  lot of space.
- Removed "stage" assertions to make sure you were using the right
  macros in the right place. Turned out to not be necessary, we'll
  employ better checks later.

core/core-packages.el

@@ -179,7 +179,6 @@ Accepts the following properties:
 Returns t if package is successfully registered, and nil if it was disabled
 elsewhere."
   (declare (indent defun))
-  (doom--assert-stage-p 'packages #'package!)
   (let ((old-plist (cdr (assq name doom-packages))))
     (when recipe
       (when (cl-evenp (length recipe))
@@ -218,21 +217,9 @@ elsewhere."
                `((add-to-list 'doom-disabled-packages ',name nil 'eq)
                  nil))))))
 
-(defmacro packages! (&rest packages)
-  "A convenience macro for `package!' for declaring multiple packages at once.
-
-Only use this macro in a module's packages.el file."
-  (doom--assert-stage-p 'packages #'packages!)
-  (macroexp-progn
-   (cl-loop for desc in packages
-            collect (macroexpand `(package! ,@(doom-enlist desc))))))
-
 (defmacro disable-packages! (&rest packages)
-  "A convenience macro like `package!', but allows you to disable multiple
-packages at once.
-
-Only use this macro in a module's packages.el file."
-  (doom--assert-stage-p 'packages #'disable-packages!)
+  "A convenience macro for disabling packages in bulk.
+Only use this macro in a module's (or your private) packages.el file."
   (macroexp-progn
    (cl-loop for pkg in packages
             collect (macroexpand `(package! ,pkg :disable t)))))