Notations for relations with explicit type arguments

We discussed this already here: https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/134

The consensus was that @{A} would be a good fit, and would not cause conflicts. I implemented it here for hopefully all relations in std++: =, ≡, ⊆, ⊂, ⊑, ##, ∈, ≡ₚ, and adopted the code at many places to make use of these notations.