Separate pseudo-royalty from extinction

[ianfab]

TODO: requires more thought regarding impact on future development

• consider playing strength implications, especially extinction eval
• 3 stages of legality pseudo-legal, static legal (current legal), legal (including uchifuzume, checkless, etc.)
• include pseudo-royal in check detection (requires non-static gives_check and legal)
• consolidate extinction and pseudo-royal eval. Evaluate king safety for every effectively royal piece. Treat extinction as weak pseudo-royalty, because it is the same, just that leaving piece in check is allowed for one half-move.
• consider interference between extinction and pseudo-royalty (and atomic captures)
  • e.g., one side has pseudo-royals and one side extinction. A piece pinned against a pseudo-royal can take an extinction piece to win. Allowed or not? (No)
• final step: turn king into a piece that is always pseudo-royal? how about pseudoRoyalCount thresholds?