Pomset utils

[eupp]

Several auxiliary definitions and lemmas for pomsets.

• lFsPreposet.Build.build to build lfspreposet from finite graph (labeling function and covering relation/Hasse diagram)
• lfsp_tseq --- topological sorting of poset
• lfsp_idx --- index of event in topological sorting
• lfsp_event --- get event by its index
• lfsposet_eqP --- reflect lemma for equality over lposets
• choiceType and countType instances for lfspreposet, lfsposet, pomset.
• other minor lemmas