Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

test/for/closure_in_body.wren

@@ -2,7 +2,7 @@ var list = []
 
 for (i in [1, 2, 3]) {
   var j = i + 1
-  list.add(fn IO.write(j))
+  list.add(fn IO.print(j))
 }
 
 for (f in list) f.call