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/class/name_inside_body.wren

@@ -1,10 +1,10 @@
 class Foo {
   static sayName {
-    IO.write(Foo.toString)
+    IO.print(Foo.toString)
   }
 
   sayName {
-    IO.write(Foo.toString)
+    IO.print(Foo.toString)
   }
 
   static toString { return "Foo!" }