String representation of the message header

String representation of the whole message, i.e. header and body

body of the message, if it has one

NYI: CLEANUP: should probably be possible for body to be
array u8, nil or a specify an LM to read from.
The question is what do we need?

get the body of the message as a String

the whole message as a sequence of bytes

header fields with all lower case

the start line of the message, for requests it is the request line, for responses the status line