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.

NEWS

@@ -46,6 +46,13 @@ New in spot 2.7.4.dev (not yet released)
     acceptance condition.  The output can be alternating only if the
     input was alternating.
 
+  - SVA's first_match operator can now be used in SERE formulas and
+    that is supported by the ltl_to_tgba_fm() translation.  See
+    doc/tl/tl.pdf for the semantics.  *WARNING* Because this adds a
+    new operator, any code that switches over the spot::op type may
+    need a new case for op::first_match.  Furthermore, the output of
+    "randltl --psl" will be different from previous releases.
+
   - The parser for SERE learned to recognize the ##n and ##[i:j]
     operators from SVA.  So {##2 a ##0 b[+] ##1 c ##2 e} is another
     way to write {[*2];a:b[+];c;1;e}, and {a ##[i:j] b} is parsed as