Extend hope_kv_list validation API with:

find_unique_presence_violations, which is a subset of validate_unique_presence,
which returns a record of found violations (a list of violating keys per
violation type field), so that a user can directly match a subset of violation
types that they care about without having to search yet another list.

validate_unique_presence keeps the same signature and semantics, but is now
implemented with find_unique_presence_violations.