💥 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.

bin/doom-doctor

@@ -332,15 +332,14 @@
         (when doom-modules
           (section! "Checking your enabled modules...")
           (let ((indent (+ indent 2)))
-            (advice-add #'require :around #'doom*shut-up)
+            (advice-add #'require :around #'doom-shut-up-a)
             (maphash
              (lambda (key plist)
                (let ((prefix (format! (bold "(%s %s) " (car key) (cdr key)))))
                  (condition-case-unless-debug ex
                      (let ((doctor-file   (doom-module-path (car key) (cdr key) "doctor.el"))
                            (packages-file (doom-module-path (car key) (cdr key) "packages.el")))
-                       (cl-loop with doom--stage = 'packages
-                                for name in (let (doom-packages
+                       (cl-loop for name in (let (doom-packages
                                                   doom-disabled-packages)
                                               (load packages-file 'noerror 'nomessage)
                                               (mapcar #'car doom-packages))
@@ -350,8 +349,7 @@
                                            (package-built-in-p name)
                                            (package-installed-p name))
                                 do (error! "%s is not installed" name))
-                       (let ((doom--stage 'doctor))
-                         (load doctor-file 'noerror 'nomessage)))
+                       (load doctor-file 'noerror 'nomessage))
                    (file-missing (error! "%s" (error-message-string ex)))
                    (error (error! "Syntax error: %s" ex)))))
              doom-modules)))))