More consistent notations for curried relations.

For example, instead of:

  Notation "( X ⊆ )"

We now use:

  Notation "( X ⊆)"

We were already doing this for = and ≡.

This solves some conflicts with the notations of MetaCoq.
2 jobs for master in 5 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #2684
coq
stdpp-coq8.5

00:02:50

passed #2685
coq
stdpp-coq8.6

00:02:29