game: reimplement parity game solving

* spot/misc/game.cc, spot/misc/game.hh: More efficient implementation
of Zielonka's algorithm to solve parity games.  Now supports SCC
decomposition and efficient handling of certain special cases.
* doc/org/concepts.org: Document "strategy" and "state-winner"
properties.
* bin/ltlsynt.cc, tests/python/paritygame.ipynb: Adjust.
* tests/core/ltlsynt.test: Add more tests.
16 jobs for next in 311 minutes and 2 seconds (queued for 3 seconds)
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