SE-0268: Refine didSet Semantics

[beccadax (Becca Royal-Gordon)]

@nuclearace

Could they not just do:

struct Thing {
  var a: Int {
    didSet(oldValue) {
      
    }
  }
}

Probably. didSet(_) should probably work too. But wherever is supposed to work, we should explicitly call it out in the proposal, change log, and release notes, and we should make sure the implementation tests that it works.