Better heuristic to estimate the utility of the Connect Four board

[hoverslam]

Some ideas:

• https://www.cs.cornell.edu/boom/2001sp/Anvari/Anvari.htm
• https://softwareengineering.stackexchange.com/questions/263514/why-does-this-evaluation-function-work-in-a-connect-four-game-in-java
• https://file.scirp.org/Html/1-9601415_90972.htm

[hoverslam]

Commit 40e2c9b improved the heuristic function.