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/method/long_name.wren

@@ -4,4 +4,4 @@ class Foo {
   }
 }
 
-IO.write((new Foo).thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result
+IO.print((new Foo).thisHasAMethodNameThatIsExactly64CharactersLongWhichIsTheMaximum) // expect: result