Provide generic hasher for enum classes

[scheibel]

Possible implementation:

template<typename EnumClass>
struct EnumClassHash
{
    std::hash<unsigned char>::result_type operator()(
        const EnumClass & arg) const
    {
        static std::hash<unsigned char> hasher;
        return hasher(static_cast<unsigned char>(arg));
    }
};

Possible usage:

template <>
struct hash<MyEnumClass> : EnumClassHash<MyEnumClass> {};

[scheibel]

I think we'll need a new submodule name for such a class (and many to follow).
It's a explicit extension to standard library concepts with the contexts hash and enum class.
Any ideas? @sbusch42 @cgcostume

[scheibel]

Seems like C++14 and 17 added support for this with their respective stdlibs. However, as we develop cppassist using C++11 I think we have to provide fallback implementations for older compilers and stdlibs.
We will have to find code to detect whether hashing is already provided by the stdlib.

[cgcostume]

is this required? and we should think about switching to C++14?

[scheibel]

I think an upgrade to C++14 becomes feasible, soon.
Fortunately, such an upgrade would imply the removal of some more implementations in cppassist.