I feel uneasy about adding methods to Optional. Optionals are mostly interacted with using overloads, operators, and syntax sugar in Swift. Correctly, every time I use map on an Optional feels like I am making the code more difficult to read for future me.
For example, an Int does not have a map method, so if I read something like counter.map { $0 + set.count } I get that counter is actually an Int?. But String, Array, etc. also have map methods and now when I am bug hunting I am reading such code much more slowly, checking if (…).map { … } should have been (…)?.map { … }.
isKnownIdentical falls even more so into this trap since every optional type for which (…).isKnownIdentical(to: …) is valid code, so too is (…)?.isKnownIdentical(to: …).
The later case ((…)?.isKnownIdentical(to: …)) may also be typed accidentally. This returns nil if the lhs is nil, even when the rhs might also be nil and thus the return value should be true. A freestanding function doesn’t suffer from this confusing call site.
If I understand Span correctly, this only works for contiguous memory, not necessarily a Dictionary or some tree structure which cannot provide a span view.