Add quiver.streams with enumerate method

I originally implemented this for a demo of one of my projects:

https://github.com/seaneagan/unscripted/blob/master/example/cat.dart

and thought it would make sense to add it back to quiver.

If quiver.iterables and quiver.async both use IndexedValue it might make sense to move it to quiver.core, but I didn't do that here.