adjust associative containers to support heterogeneous lookup

Heterogeneous lookup has been supported since C++14. To quote the linked article,

C++14 allows the lookup to be done via an arbitrary type, so long as the comparison operator can compare that type with the actual key type. This would allow a map from std::string to some value to compare against a const char* or any other type for which an operator< overload is available. It is also useful for indexing composite objects in a std::set by the value of a single member without forcing the user of find to create a dummy object (for example creating an entire struct Person to find a person by name).

Now, it's magic in that you enable it by specifying std::less<> as the third template argument to an associative container; that is:

std::map<std::string, std::string             > map1;
std::map<std::string, std::string, std::less<>> map2;

map2 supports heterogeneous lookup, whereas map1 does not.

No files, tests, or entry points are named in the issue. Start by locating the associative-container code and establish which containers are in scope; done means the relevant containers support heterogeneous lookup as described, with tests covering it.