fn part(
out: &mut impl Out,
ct: &str,
name: &str,
charset: Option<u8>,
data: &[u8],
)Expand description
One part of the body: its Content-Type (with its name, and its charset
for words), its Content-ID and Content-Location, then its bytes (WSP 8.5,
as PduComposer.makeMessageBody writes them).