A thin facade over managed_bytes[_view], offering some extra
convenience for working with strings, as well as a strong type
communicating the purpose (storing text instead of a blob).
Also introduces utils::from_hex(chunked_string_view), a fragmented
hex-decode that operates directly on a chunked_string_view without
requiring linearization. Hex pairs straddling fragment boundaries
are handled via a carry-over nibble.
