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/this/closure.wren

@@ -9,4 +9,4 @@ class Foo {
 }
 
 var closure = (new Foo).getClosure
-IO.write(closure.call) // expect: Foo
+IO.print(closure.call) // expect: Foo