Avoid describing the exact algorithm that these collections use to allocate storage. (It is a private implementation detail that we want to be able to change.)
rdar://problem/36619317
When a duplicate key is found during rehashing, it is usually either because a key was mutated after insertion, or because the dictionary itself was mutated from multiple threads at the same time.
Both of these are serious programmer errors. Promote the sanity check for duplicate keys to a full precondition and improve the error message to point out the most probable cause of the failure.
Split HashedCollections.swift.gyb into separate Set.swift and Dictionary.swift files, with some common parts going into Hashing.swift.
This is mostly a mechanical change, in preparation of unification of common parts between Set & Dictionary.
rdar://problem/34038727
