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.

doc/tl/tl.bib

@@ -216,6 +216,15 @@
   publisher	= {Springer-Verlag}
 }
 
+@Book{		  systemverilog.04.lrm,
+  title		= {SystemVerilog 3.1a Language Reference Manual:
+		  Accellera’s Extensions to Ver- ilog},
+  publisher	= {Accellera},
+  year		= {2004},
+  month		= may,
+  note		= {\url{http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.366.6206}}
+}
+
 @TechReport{	  tauriainen.03.a83,
   author	= {Heikki Tauriainen},
   title		= {On Translating Linear Temporal Logic into Alternating and