Support enif_make_resource_binary

[cigrainger]

This is in rustler_sys, but it would be nice to have it wrapped more safely in rustler.

I'd be happy to contribute this one but could use some guidance on the API. I was thinking it would make sense to add ResourceBinary to rustler::resource (though maybe it should live in rustler::types::binary?). ResourceBinary::new would take a ResourceArc and a reference to a binary with a lifetime tied to the resource object. It would hold the cloned ResourceArc and the reference to the binary as well as the size of the binary.

Does that sound reasonable? Other than lifetime annotations, are there any other considerations for ensuring the binary data is handled properly if it's external to the resource object?

[josevalim]

This can now be closed thanks to #487. :) /cc @filmor

[filmor]

Fixed in #487, you can now use resource_arc.make_binary(|inner| &inner.some_binary) to create a resource binary.