improve fuse_marks_here by detecting more patterns

This remove some restrictions that prevented fuse_marks_here from
simplifying certain patterns, as noted in the first comment of
issue #405.

* spot/twaalgos/cleanacc.cc (find_interm_rec, find_fusable): Remove
some unnecessary restrictions to singleton marks, and replace the hack
put one non-singleton mark at the beginning of the singleton list by a
sort.
* tests/python/simplacc.py: Add two test cases.
* tests/python/automata.ipynb, tests/core/remfin.test: Improve
expected results.
* NEWS: Mention the bug.
14 jobs for next in 145 minutes and 46 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
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alpine-gcc

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arch-clang

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debian-gcc-snapshot

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debian-stable-gcc

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debpkg-stable

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mingw-static

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rpm-pkg

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  Publish
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