Make `vtbackend::Function::operator()(Args...)` constexpr to return a compile-time string literal

[christianparpart]

We tend to use Function-API to construct VT sequences from a way better maintainable high level view. But this function is currently runtime-only, and it would just make sense to return a compile time string iff the input parameters are all known at compile time already.

Required knowledge: Only advanced C++ template meta programming Reference: src/vtbackend/Functions.h Signature to adapt:

    template <typename... Args>
    std::string operator()(Args&&... parameters) const