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/new.wren

@@ -1,6 +1,6 @@
 class Foo {}
 
 var foo = new Foo
-IO.write(foo is Foo) // expect: true
+IO.print(foo is Foo) // expect: true
 
 // TODO: Test precedence and grammar of what follows "new".