Adaptation of std::array for Hana.
Comparablestd::arrays are compared as per std::equal, except that two arrays with different sizes compare unequal instead of triggering an error and the result of the comparison is constexpr if both arrays are constexpr. Orderablestd::arrays are ordered with the usual lexicographical ordering, except that two arrays with different size can be ordered instead of triggering an error and the result of the comparison is constexpr if both arrays are constexpr. Foldablestd::accumulate on it, except it can be constexpr. Iterablestd::array is equivalent to iterating over it with a normal for loop.