Combining hashes

[nnnnnnnn (Nate Cook)]

@lorentey

Slice is weird in that the straightforward way to calculate the hash by sequentially combining element hashes would not be correct for slices of unordered collections like Set and Dictionary.

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.