val create : (Capnp.Message.rw Slice.t -> 'a) -> 'a t * 'a
create init is a fresh request payload and contents builder. Use one of the generated
init_pointer functions for
val create_no_args : unit -> 'a t
create_no_args () is a payload with no content.
val release : 'a t -> unit
Clear the exported refs, dropping their ref-counts. This is called automatically when you send a message, but you might need it if you decide to abort.