Harmonize std::initializer_list check
Harmonize std::initializer_list check

It's not possible to do a static check because the size is not part of the type,
but at least all checks are in one location now.

Related to #722

xtensor-stack/xsimd,JohanMabille/xsimd,JohanMabille/xsimd,xtensor-stack/xsimd