Use new polymorphism features when they arrive in Rust about time-steward

[elidupree]

I've found a way to get rid of TimeStewardLifetimedMethods and TimeStewardStaticMethods, but there are still a lot of macros that wouldn't need to be there with better polymorphism.

I believe all of these macros are currently internal, meaning that using the new polymorphism features will NOT be a breaking change to the API.

[elidupree]

Other things we might be able to improve: snapshot iterators in general (Ideally trait Snapshot should require the snapshot to be iterable, instead of having to repeat that where clause elsewhere)

[elidupree]

This issue referred to an old version of the API, so I'm closing it in favor of individual issues for the new one.