Store all temporary stuff in a specific cache folder

Instead of using effectively TMPDIR on the machine, let's use a `cache`
folder in the root of the .emacs.d directory.

modules/editor/siren-ido.el

@@ -15,8 +15,7 @@
       ido-enable-flex-matching t
       ido-enable-prefix nil
       ido-max-prospects 10
-      ido-save-directory-list-file (expand-file-name "ido.hist"
-                                                     siren-savefile-dir)
+      ido-save-directory-list-file (expand-file-name "ido.hist" siren-cache-dir)
       ido-use-faces nil
       ido-use-filename-at-point nil)
 

modules/editor/siren-smex.el

@@ -14,7 +14,7 @@
   ("C-c C-c M-x" . execute-extended-command)
 
   :config
-  (setq smex-save-file (expand-file-name ".smex-items" siren-savefile-dir))
+  (setq smex-save-file (expand-file-name "smex-items" siren-cache-dir))
   (smex-initialize))
 
 (provide 'siren-smex)

modules/editor/siren-undo-tree.el

@@ -21,7 +21,8 @@
   :config
   (global-undo-tree-mode)
   ;; autosave the undo-tree history
-  (setq undo-tree-history-directory-alist `((".*" . ,temporary-file-directory))
+  (setq undo-tree-history-directory-alist
+        `((".*" . ,(expand-file-name "undo-tree-history" siren-cache-dir)))
         undo-tree-auto-save-history t))
 
 (provide 'siren-undo-tree)