feat(tonic): Use BoxBody from http-body crate (#622)

As of `http-body` 0.4.1 its has had a `BoxBody` type similar to
`tonic::body::BoxBody`. It also has `Empty` and `Body::map_{data,err}`.

That means all the custom body things we had in tonic can basically be
replaced with that.

Note that this is a breaking change so we should merge this next time we
decide to ship a breaking release.

The breaking changes are:

- `tonic::body::Body` has been removed. I think its fine for users to
depend directly on `http-body` if they need this trait.
- `tonic::body::BoxBody` is now just a type alias for
`http_body::combinators::BoxBody<Bytes, Status>`. So the methods it
previously had are gone. The replacements are
  - `tonic::body::Body::new` -> `http_body::Body::boxed`
  - `tonic::body::Body::map_from` -> `http_body::Body::map_data` and
  `http_body::Body::map_err` depending on which part you want to map.
  - `tonic::body::Body::empty` -> `http_body::Empty`

Additionally a `Sync` bound has been added to a few methods. I actually
don't think this is a breaking change because the old
`tonic::body::Body` trait had `Sync` as a supertrait meaning the `Sync`
requirement was already there.

Fixes https://github.com/hyperium/tonic/issues/557

tonic-build/src/client.rs

@@ -37,9 +37,9 @@ pub fn generate<T: Service>(
 
             impl<T> #service_ident<T>
             where T: tonic::client::GrpcService<tonic::body::BoxBody>,
-                  T::ResponseBody: Body + HttpBody + Send + 'static,
+                  T::ResponseBody: Body + Send + Sync + 'static,
                   T::Error: Into<StdError>,
-                  <T::ResponseBody as HttpBody>::Error: Into<StdError> + Send, {
+                  <T::ResponseBody as Body>::Error: Into<StdError> + Send, {
                 pub fn new(inner: T) -> Self {
                     let inner = tonic::client::Grpc::new(inner);
                     Self { inner }

tonic-build/src/server.rs

@@ -69,7 +69,7 @@ pub fn generate<T: Service>(
             impl<T, B> Service<http::Request<B>> for #server_service<T>
                 where
                     T: #server_trait,
-                    B: HttpBody + Send + Sync + 'static,
+                    B: Body + Send + Sync + 'static,
                     B::Error: Into<StdError> + Send + 'static,
             {
                 type Response = http::Response<tonic::body::BoxBody>;
@@ -91,7 +91,7 @@ pub fn generate<T: Service>(
                                .status(200)
                                .header("grpc-status", "12")
                                .header("content-type", "application/grpc")
-                               .body(tonic::body::BoxBody::empty())
+                               .body(empty_body())
                                .unwrap())
                         }),
                     }