feat(http1): Make HTTP/1 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/1 methods and support behind an
  `http1` cargo feature, which will not be enabled by default. To use
  HTTP/1, add `features = ["http1"]` to the hyper dependency in your
  `Cargo.toml`.

src/proto/h1/dispatch.rs

@@ -5,10 +5,10 @@ use http::{Request, Response, StatusCode};
 use tokio::io::{AsyncRead, AsyncWrite};
 
 use super::{Http1Transaction, Wants};
-use crate::body::{Body, HttpBody};
+use crate::body::{Body, DecodedLength, HttpBody};
 use crate::common::{task, Future, Never, Pin, Poll, Unpin};
 use crate::proto::{
-    BodyLength, Conn, DecodedLength, Dispatched, MessageHead, RequestHead, RequestLine,
+    BodyLength, Conn, Dispatched, MessageHead, RequestHead, RequestLine,
     ResponseHead,
 };
 use crate::service::HttpService;