refactor(folding): Improve siren-folding module

- Add a siren-folding function which enables all folding-related modes,
  and change other modules to use this new function.

modules/navigation/siren-folding.el

@@ -1,38 +0,0 @@
-;;; siren-folding.el --- jimeh's Emacs Siren: folding configuration.
-
-;;; Commentary:
-
-;; Basic configuration for folding code.
-
-;;; Code:
-
-(use-package hideshowvis
-  :ensure nil ;; loaded from vendor
-)
-
-(defun siren-toggle-hiding (column)
-  "Toggle hiding/showing blocks via hs-mode.
-
-Borrowed from: http://www.emacswiki.org/emacs/HideShow"
-  (interactive "P")
-  (if hs-minor-mode
-      (if (condition-case nil
-              (hs-toggle-hiding)
-            (error t))
-          (hs-show-all))
-    (siren-toggle-selective-display column)))
-
-(defun siren-toggle-selective-display (column)
-  "Helper function for `siren-toggle-hiding'."
-  (interactive "P")
-  (set-selective-display
-   (or column
-       (unless selective-display
-         (1+ (current-column))))))
-
-;; Keybindings
-(global-set-key (kbd "C-=") 'siren-toggle-selective-display)
-(global-set-key (kbd "C-c C-h") 'siren-toggle-hiding)
-
-(provide 'siren-folding)
-;;; siren-folding.el ends here