/// PawnInfoTable::get_pawn_info() takes a position object as input, computes
/// a PawnInfo object, and returns a pointer to it. The result is also
/// stored in a hash table, so we don't have to recompute everything when
/// PawnInfoTable::get_pawn_info() takes a position object as input, computes
/// a PawnInfo object, and returns a pointer to it. The result is also
/// stored in a hash table, so we don't have to recompute everything when