Performance optimizations about mcts

[Operdies]

In many cases I think the compiler can figure out that the move constructor should be invoked, particularly if we use factory functions where the copied objects would go out of scope immediately.

When we start optimizing I will heavily advocate that we profile and test candidate versions before anything is merged into master

[Hoeiriis]

std::move() both for trees and states. Some caveats though that would need to be taken into account

[Operdies]

On this note, we should also const and nothrow anything that is guaranteed to not throw exceptions or doesn't have side effects