Fix compression (minor).

Conflicts:
	Source/standard_library/process.ml
CHERRY-PICK: We allow a "non-simple" prefix at top level of processes.
Examples: New k.New g.In(x). (P1|P2) is allowed if P1|P2
is a simple process.
&& Done:
      - Process.from_par_to_multiset replaces a process starting with
Par at top level into a multiset of processes and order them (i.e. it gives
them a unique ID)
      - Starting by accepting any action until we reach a Par, and then
apply Process.from_par_to_multiset. From now, compression and reduction
can be applied

Conflicts:
	Source/standard_library/process.ml
	Source/trace_equivalence/algorithm.ml