Transposition tables and more

[cytronicoder]

This is an ongoing development, and I will update this PR constantly.

This will close #2.

Notable changes

• Implemented a simple transposition table, favoring storage of the most recently explored nodes
  • The table can store either frequent positions or positions that take a lot of time to compute
• Implemented iterative deepening (increasing the depth of search while keeping shallow search results in transposition table) and null window search (an [alpha; beta] window of minimal size)

[cytronicoder]

We will need to create an optimized transposition table and set a lower bound: When an alpha-beta cut prunes the exploration, the returned score is the lower bound. Keeping and reusing these lower bound scores in a transposition table can help reduce the number of explored nodes.

[cytronicoder]

However, I can merge this in for now.

[cytronicoder]

We will need to create an optimized transposition table and set a lower bound: When an alpha-beta cut prunes the exploration, the returned score is the lower bound. Keeping and reusing these lower bound scores in a transposition table can help reduce the number of explored nodes.

#6 will implement this!