Add utility IsCompileTimeConstant and use it in HybridFunctor

Simon Praetoriusrequested to merge
issue/hybrid-functor-for-compiletime-constants into master

Summary

The type-trait IsIntegralConstant checks exactly for a type std::integral_constant<T,value> but does not allow any derived classes or user-defined classes with the same interface. Example of such compile-time constant is std::is_same<S,T>.

The new utility is directly used to implement HybridFunctor to allow a wider range of compiler-time input.

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