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/super/closure.wren

@@ -13,4 +13,4 @@ class Derived is Base {
 }
 
 var closure = (new Derived).getClosure
-IO.write(closure.call) // expect: Base
+IO.print(closure.call) // expect: Base