moving linear_intervals_count to hasse as iterator

[fchapoton]

to make them still faster ; also changed the moved method into an iterator as this could be convenient

follow-up of previous effort in #40171

Timings :

poset of size 4800 : 18s before ; 13 after

poset of size 5300 : 24.4 s before and17.4 s after

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