ltlsynt: improve construction of turn-based games

Improve the way transitions are duplicated when preparing the turn-based
game for synthesis. The resulting arena should now be deterministic on
nodes owned by the environment. Also move the code to another file, so
that it is easier to test (e.g. in Python).

* bin/ltlsynt.cc: move the code
* spot/twaalgos/split.cc, spot/twaalgos/split.hh: move the code and
  implement the improvements
* tests/Makefile.am, tests/python/split.py: test it
* tests/core/ltlsynt.test: update existing tests to reflect the changes
10 jobs for next in 150 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #5612
arch-clang

00:37:45

passed #5613
arch-gcc-glibcxxdebug

00:38:10

passed #5611
debian-gcc-snapshot

01:16:30

passed #5609
debian-stable-gcc

00:40:31

passed #5610
debian-unstable-gcc-coverage

00:48:48

88.9%
passed #5616
debpkg-stable

02:28:20

passed #5617
debpkg-unstable

02:27:00

passed #5614
mingw-shared

00:08:40

passed #5615
mingw-static

00:11:31

 
  Publish
passed #5618
dput
publish-unstable

00:01:40