rtlil: speed up SigSpec::sort_and_unify()

rtlil: speed up SigSpec::sort_and_unify()

std::set<> internally is often a red-black tree which is fairly
expensive to create but fast to lookup.  In the case of
sort_and_unify(), a set<> is constructed as a temporary object to
attempt to speed up lookups.  Being a temporarily, however, the cost of
creation far outweights the lookup improvement and is a net performance
loss.  Instead, sort the vector<> that already exists and then apply
std::unique().