Add enumerative strategy

[YellPika]

Implement an enumerative strategy (as is the default in SmallCheck/LeanCheck).

This seems relatively straightforward, but we should keep the following in mind:

1. We should somehow respect the weights passed to gen::next. For example, if a user writes gen::next(1, 3) then we should list sequences starting with true 3x as often as sequences starting with false.
2. We should respect whatever guarantees we settle on for Determine core function guarantees #1.