Function write_frame 

pub fn write_frame(
    message_type: u16,
    payload: &[u8],
    out: &mut Vec<u8>,
) -> Result<(), FramingError>

Encode a frame (header plus payload) onto out. The inverse of parse_frame; parse_frame(write_frame(t, p)) == (t, p).

Refuses payloads longer than MAX_PAYLOAD_LEN rather than emitting a frame the wire would reject.