[Pitch] - Add `DictionaryProtocol` to the standard library

[AlexanderM (Alexander Momchilov)]

One concrete use case I've run into is to allow customization of my group(by:) algorithm to allow callers to choose to use OrderedDictionary, or others:

@lorentey

I agree that ideally, the chainable Sequence methods would support returning an arbitrary Dictionary type, by e.g. taking the result type as an argument:

(0 ... 9).grouped(into: TreeDictionary<Int, Deque<Int>>.self) { $0 % 3 }

However, this is currently blocked on the lack of a mutable DictionaryProtocol . We now have enough examples of dictionary types to start designing dictionary protocols (likely starting by prototyping them within swift-collections), but until we get that done, we should follow the stdlib's existing practice of simply returning the canonical type, which is Dictionary in this case -- as @AlexanderM suggests in their original post.