Move the thing that prevents no_std into rpc

So, [currently][1], `async` fns/closures prevent crates from building in
a no_std environment. Since the only occurrence is this single `send`
method, I've moved it to the rpc crate.

[1]: rust-lang/rust#56974

capnp-rpc/src/lib.rs

@@ -112,6 +112,7 @@ mod sender_queue;
 mod split;
 mod task_set;
 pub mod twoparty;
+mod send_ext;
 
 pub trait OutgoingMessage {
     fn get_body<'a>(&'a mut self) -> ::capnp::Result<::capnp::any_pointer::Builder<'a>>;

capnp-rpc/src/rpc.rs

@@ -44,6 +44,7 @@ use crate::attach::Attach;
 use crate::{broken, local, queued};
 use crate::local::ResultsDoneHook;
 use crate::task_set::TaskSet;
+use crate::send_ext::SendExt;
 
 pub type QuestionId = u32;
 pub type AnswerId = QuestionId;

capnp/src/lib.rs

@@ -39,10 +39,6 @@ pub mod any_pointer;
 pub mod any_pointer_list;
 pub mod capability;
 pub mod capability_list;
-// #[cfg(not(feature = "std"))]
-mod capability_no_std;
-// #[cfg(feature = "std")]
-// mod capability_std;
 pub mod constant;
 pub mod data;
 pub mod data_list;