Add support for multiple transitions per view controller transition

[jverkoey]

Closes #31.

This is a breaking change due to removal of APIs and changing of nullability annotations on existing APIs.

This PR allows a client to associate multiple Transition instances with a single view controller transition. When a transition is initiated, each Transition instance will be provided the same transition context.

This allows us to build smaller transition types, such as "FadeTransition" or "SlideTransition" which each accept a target view. These transitions can then be combined to create a "slide and fade in transition" like so:

modalViewController.transitionController.transitions = [
  FadeTransition(target: .foreView),
  SlideUpTransition(target: .foreView)
]

Some open questions this change has introduced:

• How does view duplication work across transitions? Ideally if a view were duplicated by one transition it would not be re-duplicated by another, but ensuring that all transitions access views in a safe manner would require adoption of a convention, e.g. context.resolvedView(for: view), which would be routed through a shared view duplicator.
• Who is responsible for associating transitions with a given view controller transition? There are potentially three actors who might be interested in associating Interaction instances. This topic is discussed in Add support for on-demand transitions #39.
• How might a client easily build a transition that is composed of other transitions? This may warrant a follow-up feature change, possibly with the introduction of a TransitionWithChildTransitions protocol that allows the parent transition to return an array of child transitions.