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/object/to_string.wren

@@ -1,2 +1,2 @@
 class Foo {}
-IO.write((new Foo).toString) // expect: <object>
+IO.print((new Foo).toString) // expect: <object>