Add Transposition tables to the tree search

[giewev]

Should use a hash table to lookup previously calculated information about a position. This may include:

• The best move(s) that were chosen for this position
• The score given to this position
• The depth to which this position was searched when generating the above info

[giewev]

Currently the lookup is used under 2 conditions during alpha-beta:

1. If the transposition is from an equally deep or deeper search than the current one, simply return that result instead
2. If the transposition is known but not deep enough, use the best move from the transposition as the first move to try