Change `SequenceOf`/`SetOf` behaviour

- New elements to `SequenceOf`/`SetOf` objects can now be added at any
  position - the requirement for the new elements to reside at the end
  of the existing ones (i.e. s[len(s)] = 123) is removed.

- Removed default initializer from `SequenceOf`/`SetOf` types to ensure
  consistent behaviour with the rest of ASN.1 types. Before this change,
  `SequenceOf`/`SetOf` instances immediately become value objects
  behaving like an empty list. With this change, `SequenceOf`/`SetOf`
  objects remain schema objects unless a component is added or
  `.clear()` is called.

- Added `.reset()` method to all constructed types to turn value object
  into a schema object.