Remove exhashmap<T> class.

Remove exhashmap<T> class.

Class exhashmap<T> was a workaround for missing std::hash_map<Key, T>
in the original C++98 standard. It was put in GiNaC because map<Key, T>
was deemed too slow. Since C++11 there is std::unorderd_map<Key, T>,
which is hash-based. To be able to use it, add specializations of
std::hash<ex> and std:equal_to<ex>.