Consider refactoring `Frontier(BaseFork)` into scoped mixins

[fselmo]

Frontier is growing infinitely as the first BaseFork implementation class. This is a really powerful class that gives us the flexibility of making fork-based decisions in testing. I think the complexity of this class and what it does will only keep growing.

We should consider abstracting out some of the logic in this class - perhaps into fine-scoped mixin classes with focused separation of concerns. See this comment for a bit of extra context.