Completely rewrite the internals and most of the APIs.

[zach-klippenstein]

• All the actual transition logic is moved out of the Backstack composable and into TransitionController.
• BackstackInspector and InspectionGestureDetector are replaced with a single API in a new, dedicated module: xrayed().
• Backstack now just handles the container and managing saveable state.
• Introduces FrameController, an abstraction which can be used to implement both transitions, the inspector functionality, and things like #17.
• Removes the spring animations from the inspector. They added unnecessary lag to the gestures.
• Adds more tests for state handling.
• Fixed some races that caused navigation animation glitches.

This change fixes a number of issues:

• Closes #4
• Closes #5
• Closes #16
• Closes #42
• Closes #44 (camera distance API is used to improve the 3D view, but not for zooming itself)
• Unblocks #17