Add get_all_histories

[jhtschultz]

Addresses Issue #401. As requested, GetAllHistories() returns a std::vector<std::unique_ptr<State>>.

Use State::FullHistory to check for equality, rather than State::ToString

Is there any reason we need to actually perform this check for equality or will a DFS suffice? I couldn’t come up with a scenario in which there’s a difference so I implemented it as a straightforward DFS. Wanted to check though because there’s been some discussion around this issue and I could well have missed something.

I gave GetAllHistories() its own file. Seemed like the right way to go but if there’s a preference for including it in the get_all_states file as a separate function I’m happy to do that.

And if there’s anything else I can do for this PR just let me know :)

[gabrfarina]

I'm not a maintainer, but I think there was consensus that the code that counts infosets in open_spiel/examples/count_all_states.cc would need to move to GetAllHistories to avoid undercounting in some classes of games (#399).

Thanks for this, once it lands I will move the tests in #376 to use it :-)

[jhtschultz]

That ought to do it. Thanks @gabrfarina!

[lanctot]

Oh hey John, seems like there is a conflict with master, can you double check that your last commit was included in the update along with the rest?

[lanctot]

Just checked, it was not included. Sorry, completely missed that (I modified the same file internally.)

But also: I don't think we want to replace states entirely with histories in that example because that would overcount states in the perfect information games, right?

Edit: just noticed the example doesn't actually count the states! It probably should... given the file name. ;-) I can do both before next update (Tuesday).