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/mod.rs

@@ -1,7 +1,8 @@
 use bytes::BytesMut;
 use http::{HeaderMap, Method};
 
-use crate::proto::{BodyLength, DecodedLength, MessageHead};
+use crate::body::DecodedLength;
+use crate::proto::{BodyLength, MessageHead};
 
 pub(crate) use self::conn::Conn;
 pub use self::decode::Decoder;
@@ -11,7 +12,6 @@ pub use self::io::Cursor; //TODO: move out of h1::io
 pub use self::io::MINIMUM_MAX_BUFFER_SIZE;
 
 mod conn;
-pub(super) mod date;
 mod decode;
 pub(crate) mod dispatch;
 mod encode;