Fix constexpr accepts() implementation to be C++11 compliant (#452)
In C++11, `constexpr` functions must not contain compound statements.
This change makes the implementation of `message_packet::accepts` use
a single conjunction instead of a `switch` statement.
See https://gcc.godbolt.org/z/zKbsx3nY5.
Co-authored-by: John Wellbelove <jwellbelove@users.noreply.github.com>
