Adaptation of std::integer_sequence for Hana.
Comparablestd::integer_sequences are equal if and only if they have the same number of elements, and if corresponding elements compare equal. The types of the elements held in both integer_sequences may be different, as long as they can be compared. Foldableinteger_sequence is equivalent to folding a sequence of std::integral_constants with the corresponding types. Iterableinteger_sequence is equivalent to iterating over a sequence of the corresponding std::integral_constants. Searchableinteger_sequence is equivalent to searching through the corresponding sequence of std::integral_constants.