Add support for binding NSFastEnumeration to IEnumerable

Here's a use case: https://stackoverflow.com/questions/10057343/binding-an-obj-c-type-implementing-the-nsfastenumeration-protocol-to-monotouch

Ref: https://web.archive.org/web/20170630211645/https://bugzilla.xamarin.com/show_bug.cgi?id=4391

We've annotated some places that would benefit from this with `INSFastEnumeration`:

https://github.com/search?q=repo%3Adotnet%2Fmacios%20INSFastEnumeration&type=code