Make session persistence module agnostic

They've been removed from feature/workspaces and moved into
core/autoload/sessions, which falls back to desktop.el if persp-mode
isn't present. This also offers a substantial speed up to
restart+restoring and restoring sessions in general.

Also fixes #1210, where the newly spawned frame after doom/restart
wasn't focused.

Introduces the following commands:

- doom/restart
- doom/restart-and-restore
- doom/quickload-session
- doom/quicksave-session
- doom/load-session
- doom/save-session
- +workspace/restore-last-session (alias for doom/quickload-session)

And removes

- +workspace/load-session
- +workspace/save-session
- +workspace/load-last-session (renamed to +workspace/restore-last-session)
- +workspace/restart-emacs-then-restore (replaced by doom/restart-and-restore)
- :ss (ex command)
- :sl (ex command)

core/core.el

@@ -253,6 +253,9 @@ and `doom-exit-window-hook'."
  async-byte-compile-log-file  (concat doom-etc-dir "async-bytecomp.log")
  auto-save-list-file-name     (concat doom-cache-dir "autosave")
  backup-directory-alist       (list (cons "." (concat doom-cache-dir "backup/")))
+ desktop-dirname              (concat doom-etc-dir "desktop")
+ desktop-base-file-name       "autosave"
+ desktop-base-lock-name       "autosave-lock"
  pcache-directory             (concat doom-cache-dir "pcache/")
  request-storage-directory    (concat doom-cache-dir "request")
  server-auth-dir              (concat doom-cache-dir "server/")