struct EnumClassHash

Hash for enum types, to be used instead of std::hash

<T

> when T is an enum.

Until C++14, std::hash

<T

> is not defined if T is a enum (see http://www.open-std.org/jtc1/sc22/wg21/docs/lwg-defects.html#2148). But even with C++14, as of october 2016, std::hash for enums is not widely implemented by compilers, so here when T is a enum, we use EnumClassHash instead of std::hash. (For instance, in b3d::hash_combine(), or b3d::UnorderedMap.)