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/h2/mod.rs

@@ -9,8 +9,7 @@ use pin_project::pin_project;
 use std::error::Error as StdError;
 use std::io::IoSlice;
 
-use super::DecodedLength;
-use crate::body::HttpBody;
+use crate::body::{DecodedLength, HttpBody};
 use crate::common::{task, Future, Pin, Poll};
 use crate::headers::content_length_parse_all;