Convert a slice of Ids to references at zero-cost

[SSheldon]

There used to be an IdSlice trait for doing this, but it was removed in SSheldon/rust-objc@4118e96. This isn't possible until rust-lang/rust#5016 is completed, because until then an Id is a different size than a reference.

[syntheorem]

Embedded drop flags are finally gone! Not sure whether the change is in stable yet, but just letting you know that you could likely revisit this now :)