contract inference for std map from string to string

[qqiangwu]

std::map<std::string, std::string> m;
const auto& s1 = m.at("key");
// pset(s1) == pset("key")

const auto& s2 = m["key"];
// pset(s2) == pset("key")

[qqiangwu]

FD->getQualifiedNameAsString()

std::map<int, int>::at