Optimizer - Constant propagation for variables

* It doesn't work for function arguments at this point, but shouldn't be
too hard to implement that.
* Can't use it to its full potential mostly because I need to get rid of
the node-specific shit in the AST.

test/build.srd

@@ -4,7 +4,9 @@
 		(1920 1080)
 	)
 	(optimizer on
-		(constant-folding on)
+		(constant-folding on
+			# (fixed-resolution on)
+		)
 		(constant-propagation on)
 	)
 )