`raw.hpp`: missing check for `MAX_NUM_ARRAY_ELEMENTS` in the `std::array` pack/unpack versions.

[greg7mdp]

Contrary to what I thought at first, the enable_if conditions in raw.hpp custom traits like is_trivial_array_element<T> cannot be replaced by standard C++ traits like std::is_trivially_copyable_v<T>, so just add the missing MAX_NUM_ARRAY_ELEMENTS check, and update the code to use C++20 concepts rather than std::enable_if.