I wrote a test for capturing en passant that revealed some bugs. Cool!
Implement the missing part of move validation that checks for an en passant move.
Implement PositionBuilder::to_move() to set player_to_move.
The most difficult part of this fix was finding the logic error in
Move::is_en_passant(). Instead of checking for non-zero, check for equality against
the en passant flag value. Checking for non-zero was causing this method to return
true in the double push case.
