Add trait to get binary size of messages

[wucke13]

If a message contains only known size types (e.g. arrays and primitives, but no Vec etc.) it would be cool to have a const helper function that determines the size of a buffer that is guaranteed to fit the serialized message. This greatly improves the usability in environments without alloc.

[sempervictus]

Finding this to be somewhat necessary rather than a convenience when using typed enums getting their type from outside their own byte range (typed body getting type passed as ctx from the header) - can't convert those to bytes for measurement conveniently and need to edit them destructured which then has to update the header length.