feat(emacs.d): Add functions & bindings to manage global font size

This makes it possible to quickly adjust the size of text in all
frames using one keyboard shortcut. Each of these functions
understands a prefix argument to mean "please only operate on the
current buffer", hence the following bindings and effects:

Global:
* `C-=`: Increase the global font size (chosen because `+` lies on the
  `=` key)
* `C--`: Decrease the global font size
* `C-x C-0`: Restore the global default font size

Local:
* `C-u C-=`: Increase the local font size
* `C-u C--`: Decrease the local font size
* `C-u C-x C-0`: Restore the local default font size

tools/emacs/config/functions.el

@@ -1,5 +1,3 @@
-(require 's)
-
 (defun load-file-if-exists (filename)
   (if (file-exists-p filename)
       (load filename)))
@@ -219,4 +217,34 @@
                       (getenv "USER"))
                     todo))))
 
+;; Custom text scale adjustment functions that operate on the entire instance
+(defun modify-text-scale (factor)
+  (set-face-attribute 'default nil
+                      :height (+ (* factor 5) (face-attribute 'default :height))))
+
+(defun increase-default-text-scale (prefix)
+  "Increase default text scale in all Emacs frames, or just the
+  current frame if PREFIX is set."
+
+  (interactive "P")
+  (if prefix (text-scale-increase 1)
+    (modify-text-scale 1)))
+
+(defun decrease-default-text-scale (prefix)
+  "Increase default text scale in all Emacs frames, or just the
+  current frame if PREFIX is set."
+
+  (interactive "P")
+  (if prefix (text-scale-decrease 1)
+    (modify-text-scale -1)))
+
+(defun set-default-text-scale (prefix &optional to)
+  "Set the default text scale to the specified value, or the
+  default. Restores current frame's text scale only, if PREFIX is
+  set."
+
+  (interactive "P")
+  (if prefix (text-scale-adjust 0)
+    (set-face-attribute 'default nil :height (or to 120))))
+
 (provide 'functions)