Introduce general.el & rewrite map!

+ Now uses an overriding keymap for leader keys, so that it is always
  available, even outside of normal/visual states. In insert/emacs
  states, or in sessions where evil is absent, an alternative prefix is
  used for leader/localleader keys. See these variables:
  + doom-leader-prefix
  + doom-leader-alt-prefix
  + doom-localleader-prefix
  + doom-localleader-alt-prefix
+ Keybinds now support alternative prefixes through the new :alt-prefix
  property. This is useful for non-evil users and non-normal evil
  states. By default, this is M-SPC (leader) and M-SPC m (localleader).
+ Removed +evil-commands flag from config/default (moved to
  feature/evil/+commands.el).
+ config/default/+bindings.el has been split into
  config/default/+{evil,emacs}-bindings.el, which one is loaded depends
  on whether evil is present or not. The latter is blank, but will soon
  be populated with a keybinding scheme for non-evil users (perhaps
  inspired by #641).
+ The define-key! macro has been replaced; it is now an alias for
  general-def.
+ Added unmap! as an alias for general-unbind.
+ The following modifier key conventions are now enforced for
  consistency, across all OSes:
    alt/option      = meta
    windows/command = super
  It used to be
    alt/option      = alt
    windows/command = meta
  Many of the default keybinds have been updated to reflect this switch,
  but it is likely to affect personal meta/super keybinds!

The map! macro has also been rewritten to use general-define-key. Here
is what has been changed:

+ map! no longer works with characters, e.g. (map! ?x #'do-something) is
  no longer supported. Keys must be kbd-able strings like "C-c x" or
  vectors like [?C-c ?x].
+ The :map and :map* properties are now the same thing. If specified
  keymaps aren't defined when binding keys, it is automatically
  deferred.
+ The way you bind local keybinds has changed:

    ;; Don't do this
    (map! :l "a" #'func-a
          :l "b" #'func-b)
    ;; Do this
    (map! :map 'local "a" #'func-a
                      "b" #'func-b)

+ map! now supports the following new blocks:
  + (:if COND THEN-FORM ELSE-FORM...)
  + (:alt-prefix PREFIX KEYS...) -- this prefix will be used for
    non-normal evil states. Equivalent to :non-normal-prefix in general.
+ The way you declare a which-key label for a prefix key has changed:

    ;; before
    (map! :desc "label" :prefix "a" ...)
    ;; now
    (map! :prefix ("a" . "label") ...)

+ It used to be that map! supported binding a key to a key sequence,
  like so:

    (map! "a" [?x])  ; pressing a is like pressing x

  This functionality was removed *temporarily* while I figure out the
  implementation.

Addresses: #448, #814, #860
Mentioned in: #940

core/core-lib.el

@@ -369,34 +369,6 @@ For example:
          ,(doom--resolve-path-forms spec '--directory--))
     (doom--resolve-path-forms spec)))
 
-(defmacro define-key! (keymaps key def &rest rest)
-  "Like `define-key', but accepts a variable number of KEYMAPS and/or KEY+DEFs.
-
-KEYMAPS can also be (or contain) 'global or 'local, to make this equivalent to
-using `global-set-key' and `local-set-key'.
-
-KEY is a key string or vector. It is *not* piped through `kbd'."
-  (declare (indent defun))
-  (or (cl-evenp (length rest))
-      (signal 'wrong-number-of-arguments (list 'evenp (length rest))))
-  (if (and (listp keymaps)
-           (not (eq (car-safe keymaps) 'quote)))
-      `(dolist (map (list ,@keymaps))
-         ,(macroexpand `(define-key! map ,key ,def ,@rest)))
-    (when (eq (car-safe keymaps) 'quote)
-      (pcase (cadr keymaps)
-        (`global (setq keymaps '(current-global-map)))
-        (`local  (setq keymaps '(current-local-map)))
-        (x (error "%s is not a valid keymap" x))))
-    `(let ((map ,keymaps))
-       (define-key map ,key ,def)
-       ,@(let (forms)
-           (while rest
-             (let ((key (pop rest))
-                   (def (pop rest)))
-               (push `(define-key map ,key ,def) forms)))
-           (nreverse forms)))))
-
 (defmacro load! (filename &optional path noerror)
   "Load a file relative to the current executing file (`load-file-name').