SE-0453: Vector, a fixed size array

[Karl]

@xwu

These are pretty obviously useful. The point I want to make here is that expand and unfold are clearly being used as terms-of-art and—given that other noncopyable collections are probably going to pop up sooner or later—we should be happy with their naming here as they're undoubtedly going to be used as precedent.

Hm? These seem to me like stopgaps, providing the equivalent of:

extension Array {
  init(_ elements: some Sequence<Element>)
}

except that Sequence doesn't support non-copyable Element types or throwing, so it uses a closure instead.

I expect the new collection protocols will support both of these things, at which point the initialisers become redundant. So I don't think these will be used as precedent.

Speaking of which, initialisation from a sequence seems useful, but I couldn't find it mentioned in the proposal. The SIMD types have such an initialiser, and I think it makes sense on Vector as well.