Adapt abbreviation algorithm to not visit complete mismatch structures

[philomates]

I was testing the experimental abbreviation feature on a list of datomic entities and noticed some new exceptions.
Datomic entities are tricky to work with in the presence of clojure.walk functions because you can't assoc/cons onto them. Generally matcher-combinators doesn't work too well on them but because of this.

In this particular case, I realized that it was because the walk was descending into missing/unexpected data, which shouldn't be abbreviated at all. If that data is a datomic entity, it raises an exception. So to avoid such exceptions, and data traversal, I added a patched version of prewalk that allows not descending into elements that satisfy a predicate.