Formalize the need for FalsifiedTime args in retrofitted Collection methods in NodeSet

[fiveham]

Currently, there are duplicate methods in NodeSet, such as clear() and clear(FalsifiedTime), where one method fulfills a requirement of the Set or Collection interface and the other fulfills the practical necessity of providing graph-modifying methods access to their appropriate modification-point on the growing time-tree of solution events.

Using Collection's methods in principle while never using Collection's methods themselves clashes with the concept of Collection as an API. NodeSet should be reconceived such that

• NodeSet retains its analogous methods that accept a FalsifiedTIme and loses its timeless methods from Collection
• the analogous methods that accept a FalsifiedTime are removed and Collection's methods are used instead, where the pertinent FalsifiedTime is specified elsewhere, such as on a third-party stack, or
• new analogous methods are used that do not accept a FalsifiedTime and return a FalsifiedTime describing the solution events that occurred between entering that method and returning from it.

[fiveham]

FalsifiedTime-arg methods have been removed. They were needed only to properly attribute causation of cascading solution events. Cascading solution events have been removed because they violate separation of concerns by tying the solution process intimately into the model of the state of the puzzle.