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/editor/siren-folding.el

@@ -0,0 +1,50 @@
+;;; siren-folding.el --- jimeh's Emacs Siren: folding configuration.
+
+;;; Commentary:
+
+;; Basic configuration for folding code.
+
+;;; Code:
+
+(use-package hideshow
+  :ensure nil ;; loaded from emacs built-ins
+  :demand
+
+  :bind
+  ("C-=" . siren-folding-toggle-selective-display)
+  ("C-c C-h" . siren-folding-toggle)
+
+  :init
+  (defun siren-folding (&optional arg)
+    "Activate or deactivate code folding.
+Optional ARG is passed directly to mode toggle function."
+    (hs-minor-mode (or arg t))
+    (hideshowvis-minor-mode (or arg t)))
+
+
+  (defun siren-folding-toggle (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-folding-toggle-selective-display column)))
+
+  (defun siren-folding-toggle-selective-display (column)
+    "Helper function for `siren-folding-toggle'."
+    (interactive "P")
+    (set-selective-display
+     (or column
+         (unless selective-display
+           (1+ (current-column)))))))
+
+(use-package hideshowvis
+  :ensure nil ;; loaded from vendor
+  :demand)
+
+(provide 'siren-folding)
+;;; siren-folding.el ends here