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/closure/closed_closure_in_method.wren

@@ -4,7 +4,7 @@ var foo = null
   var local = "local"
   class Foo {
     method {
-      IO.write(local)
+      IO.print(local)
     }
   }