Let Optional, Dictionary and Array conditionally conform to Hashable

[lorentey (Karoy Lorentey)]

Strong +1; types in the standard library should conform to Hashable whenever possible.

Here are some additional types that could benefit from conditional Hashable conformance, in order of decreasing (subjective) importance:

• Range and the partial range types
• Slice, with caveat below
• DictionaryLiteral
• CollectionOfOne
• EmptyCollection

(I expect the last two would pop up mostly when they're used as constrained type parameters of generic types.)

Slice supports unordered collections, so if we want to make it Hashable, we need to decide if lexicographic equality and ordered hashing is suitable for it. @nnnnnnnn has a convincing argument that it is:

Combining hashes

I think slices of sets and dictionaries probably need to be order-dependent when we compare them. Those two types are "substitutable" for their primary use cases when equal, but definitely not for slicing operations.

Indeed, a Set.SubSequence value is decidedly not substitutable with slices from other sets, even if they happen to contain the same elements.