Now size() returns the total number of elements, and sizes() returns an
N-dimensional vector containing each size separately.

Let’s see how it goes with a very flat hierarchy and no .h extension.

Use size_t instead of ptrdiff_t and reduce header usage.

This code is not of very good quality, but a few methods such as the
RGB/HSV/HSL/YUV conversions are still useful, so I’m deciding to keep
them for now.

This is because <cstdint> is not guaranteed to expose C99 types to the
global namespace, and we don’t want to go through the hassle of using
each of them one by one.