implement SVA's first_match operator
* NEWS, doc/tl/tl.tex, doc/tl/tl.bib: Document it.
* spot/parsetl/parsetl.yy, spot/parsetl/scantl.ll: Parse it.
* spot/tl/formula.cc, spot/tl/formula.hh, spot/tl/dot.cc,
spot/tl/mutation.cc, spot/tl/print.cc, spot/tl/randomltl.cc,
spot/twaalgos/ltl2tgba_fm.cc: Adjust to support first_match.
* spot/tl/mark.cc, spot/tl/simplify.cc, spot/tl/snf.cc,
spot/tl/unabbrev.cc, spot/twa/formula2bdd.cc,
spot/twaalgos/ltl2taa.cc: Ignore it.
* tests/core/acc_word.test, tests/core/randpsl.test: Add more tests.
* tests/core/rand.test, tests/core/unambig.test,
tests/python/randltl.ipynb: Adjust.
* tests/python/formulas.ipynb: Show first_match.
6fac026454