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-os.el

@@ -20,9 +20,13 @@
   (when (featurep 'exec-path-from-shell)
     `(exec-path-from-shell-copy-envs ,@vars)))
 
+;; key conventions:
+;;   alt/option      = meta
+;;   windows/command = super
+
 (cond (IS-MAC
-       (setq mac-command-modifier 'meta
-             mac-option-modifier  'alt
+       (setq mac-command-modifier 'super
+             mac-option-modifier  'meta
              ;; sane trackpad/mouse scroll settings
              mac-redisplay-dont-reset-vscroll t
              mac-mouse-wheel-smooth-scroll nil
@@ -60,11 +64,16 @@
 
       (IS-LINUX
        (setq x-gtk-use-system-tooltips nil    ; native tooltips are ugly!
-             x-underline-at-descent-line t))  ; draw underline lower
+             x-underline-at-descent-line t    ; draw underline lower
+             x-alt-keysym 'meta))
 
       (IS-WINDOWS
-       (setq w32-get-true-file-attributes nil) ; fix file io slowdowns
-       ))
+       (setq w32-get-true-file-attributes nil  ; fix file io slowdowns
+             ;; map window keys to super
+             w32-pass-lwindow-to-system nil
+             w32-lwindow-modifier 'super
+             w32-pass-rwindow-to-system nil
+             w32-rwindow-modifier 'super)))
 
 (provide 'core-os)
 ;;; core-os.el ends here