2.2.0

Extends hope_kv_list validation API.

Adds 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.