Tweaks around when to use heap storage for properties.
Tweaks around when to use heap storage for properties.

- Test fields.count first (faster).
- Update the comment to call out how the use of oneof{} makes the current
  check not completely correct.

allevato/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,apple/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,thomasvl/swift-protobuf,allevato/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,cburrows/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf,allevato/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,thomasvl/swift-protobuf,apple/swift-protobuf,allevato/swift-protobuf,cburrows/swift-protobuf,tbkka/swift-protobuf,apple/swift-protobuf,thomasvl/swift-protobuf,tbkka/swift-protobuf