Hashable
Dictionary.Keys
CollectionOfOne
EmptyCollection
Adds `Hashable` conformance to `Dictionary.Keys`, `CollectionOfOne` and `EmptyCollection`. _Proposal: [SE-0514](https://github.com/swiftlang/swift-evolution/blob/main/proposals/0514-hashable-conformance-for-dictionarykeys-collectionofone-emptycollection.md)_