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/field/default_to_null.wren

@@ -1,5 +1,5 @@
 class Foo {
-  write { IO.write(_field) }
+  write { IO.print(_field) }
 }
 
 (new Foo).write // expect: null