Add best::bind, best::row::select(), allow some bytes.h functions in constexpr

[mcy]

This change focuses on some improvements needed by reflection. These are:

• best::row::query(), which queries a row for specific elements based on their type (and BestRowKey).
• best::bind, a tag that changes best::row and best::option's deduction guides. best::row{best::bind, a, b, c} will always deduce a row of references, similar to std::tie().
• best::compare_bytes and best::search_bytes are constexpr for best::span<char> et. al.
• Changed the implementation details banner.
• Moved all of the best::span function definition out of the class, since it was pretty cluttered.