fixed pawns being able to move two spaces forward after first move
"Fixed" an interesting bug that was first detected as pawns being
unable to eat diagonally. Previously, moving a piece didn't update a
piece's internal pos. Fixing this wasn't quite enough, because
ChessBoard.moveRevealsCheck move pieces with move and replaced them
with place.

To prevent future bugs of this variety, I recommend modifying
ChessBoard.move to return a new ChessBoard instance, rather than
mutating itself.