Turn iProp into a notation.
This avoids Coq distinguishing iProp and uPred (iResUR _) when it should not.
This commit not only solved Janno's problem in merge request !9 (merged), but also solved an issue in Amin's code. There, during some proofs, we had to
changesome occurrences of
iPropbecause both types appeared in the implicit arguments of
length, which in turn made