Combining hashes

[gwendal.roue (Gwendal Roué)]

@lorentey

I think we should have Array conform to Hashable when its Element type does. In fact, many of the stdlib types could be candidates for conditional Hashable conformance:

This is currently discussed at Let Optional, Dictionary and Array conditionally conform to Hashable - #19 by Morten_Bek_Ditlevsen. There even is a WIP implementation.