is tag_invoke actually needed?

[vinniefalco]

With wg21 moving away from the tag_invoke idiom, we might re-evaluate whether tag_invoke is actually necessary in Buffers. Instead, we could use member functions. This might be problematic though. buffers::front is necessarily a free function. Whether or not an algorithm is a free function or a member would then vary by algorithm, which impairs ergonomics. An analysis is necessary.