[Second review] SE-0453: Vector, a fixed size array

[Karl]

@lorentey

Vector will work well at holding data at rest -- serving as its storage representation. I expect it will be generally less useful for representing data in transit, because it is so specific: a function taking a vector can only ever be called with one, potentially forcing the caller to copy its data into a new temporary vector just to be able to invoke it.

And when they need to copy their data in to a Vector, they'll find the expected Vector<let N: Int, Element>.init(_: some Sequence<Element>) does not exist.

@Karl

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.

And when they get a Vector out of a function and need to copy it in to an Array or something, they'll find the expected RangeReplaceableCollection.init<let N: Int>(_: Vector<N, Element>) also does not exist.

So we definitely don't make it seamless for people to write functions taking/returning vectors.