Function encode 

pub fn encode(
    fields: &[(&str, &Value)],
    out: &mut Vec<u8>,
) -> Result<(), DocBodyError>

Encode fields as a document body container, appending bytes to out.

Fields are written in iteration order. Duplicate field names are allowed (deduplication is the caller’s responsibility).

Returns an error only if fields.len() > 65535 or a field name is longer than 65535 bytes; both are pathological in practice.