Replace the bare std::set<sstring>/std::map<sstring, std::set<sstring>>
member types with named aliases that use std::less<> as the comparator.
The transparent comparator enables heterogeneous lookup with
string_view keys.
This commit is a pure refactor with no behavioral change: the parser
return types, constructor parameters, observer template instantiations,
and start_audit() locals are all updated to use the aliases.
724b9e66ea