Get rid of overlapping notations for Par.

Par is now defined as an expression of type [∀ X, expr X] (instead of a
value) and we prove that it is stable under weakening and substitution.
2 jobs for master
Status Job ID Name Coverage
  Test
passed #360
coq
buildjob

00:03:24

failed #340
coq
buildjob

00:03:07