Allow for sharing on an `email_body` across multiple `Envelope_with_next_hop.t`'s

`type t````=` `{`

`envelopes : Async_smtp_types__.Envelope_bodiless_routed.t list;`
`email_body : Email_message.Email.Raw_content.t;`

`}```
`include sig ... end`
`val email_body : t ‑> Email_message.Email.Raw_content.t`
`val envelopes : t ‑> Async_smtp_types__.Envelope_bodiless_routed.t list`
`module Fields : sig ... end`
`val sexp_of_t : t ‑> Base.Sexp.t`
`val single_envelope : Async_smtp_types__.Envelope_routed.t ‑> t`
`val to_envelopes : t ‑> Async_smtp_types__.Envelope_routed.t list`