Determine core function guarantees

[YellPika]

In order to write correct generators, users need to understand what should be guaranteed by the core operations.

Current status:

• gen::choose(n)
  • Only returns values <= n
  • For any infinite set of calls, at least one will return 0
    • Satisfied by an exhaustive strategy
    • Almost-certainly satisfied by a random strategy
• gen::succeed()
  • May not return (this is only a user hint)
• gen::shrink(n)
  • Only returns values < n
• gen::label(id)
  • Does nothing by itself (only affects the behavior of other effects)

Once this is worked out these guarantees should be properly documented.