Make tree multithread-friendly

[RedKinda]

Wraps the tree variable in a Mutex, so that execute only requires &self instead of &mut self. It also drops the lock while PP::playout is running, since that is the CPU intensive part. This allows for multiple threads to be running execute on it at the same time.

[Sagebati]

Hi, thanks for your PR.

Adding multi threading was of course on my plan, I'm happy someone is experimenting with this library.

I'm interested if you got some benchmark without your parallel implementation.

[RedKinda]

Here is the (mess of a) project im using it in https://github.com/RedKinda/generalsio/blob/master/rs/src/mcts.rs

In the multithreaded version I am getting ~9k rollouts per second at maximum of 200 turns in one rollout, at 15 worker threads. For the old version i used to get around ~1600 rollouts per second, at 200 max turns as well, both on my local machine.

It is possible there is a faster way to do this, maybe with a multithread-friendly tree, this was a quick and simple way to get a good performance boost.