Use signed sizes/indices instead of unsigned integer type as size_t

[ganmodokix]

https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1428r0.pdf

[ganmodokix]

e.g., std::ssize() instead of .size()

[ganmodokix]

replace std::size_t with std::ptrdiff_t

[ganmodokix]

partly merged at #88