widgets: Further compact some of the internal representations

Many widgets adopted the _im(mutable) idiom to allow us to reduce the
RAM overhead of each widget. Where the immutable data considers entirely
of integers smaller than 256 then we can push this just a little further
by switching from a tuple to a bytes.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

wasp/widgets.py

@@ -305,7 +305,7 @@ class Checkbox():
 class GfxButton():
     """A button with a graphical icon."""
     def __init__(self, x, y, gfx):
-        self._im = (x, y)
+        self._im = bytes((x, y))
         self.gfx = gfx
 
     def draw(self):
@@ -410,7 +410,7 @@ class Spinner():
     widget and requires 60x120 px.
     """
     def __init__(self, x, y, mn, mx, field=1):
-        self._im = (x, y, mn, mx, field)
+        self._im = bytes((x, y, mn, mx, field))
         self.value = mn
 
     def draw(self):