Providing instant_roll and booked_roll

[cusma]

I see. I think I will take the best of two worlds in a future PR, refactoring the methods as follows:

• Turning dice_rolls into an internal subroutine;
• Provide a instant_roll, which takes the round a player want as method input;
• Provide a booked_roll, which takes the round a player as previously committed to.

Originally posted by @cusma in #1 (comment)

[fabrice102]

Yes, that's a good option. It is also important to make it clear in the README and contract metadata that the contract should only be called once for a given round, using either instant_roll or booked_roll (even for a different set of dices).
With a counter-example.