game: git rid of the parity_game class
This class was a simple wrapper on top of twa_graph_ptr, but it's easier to simply use a twa_graph_ptr with a "state-player" property instead, this way we will be able to modify the automata I/O routines to support games directly. * spot/misc/game.cc, spot/misc/game.hh: Rewrite the solver and pg_printer interface. * bin/ltlsynt.cc: Adjust. * NEWS: Mention this change. * doc/org/concepts.org: Mention the state-player property.