[swift2objc] Support optional primitives

[liamappelbe]

Optional object types are straightforward to support, because they map to nullable object pointers in ObjC. But for optional primitive types like Int?, there's no ObjC equivalent. So the swift compiler will raise an error if you try to annotate an entity involving Int? with @objc.

The straightforward solution is to box the Int in an object.

Dart does support int?, so ideally we'd also unbox it on the Dart side. To do this we'd need to special case the box type in ffigen.