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/common/buf.rs

@@ -21,6 +21,7 @@ impl<T: Buf> BufList<T> {
     }
 
     #[inline]
+    #[cfg(feature = "http1")]
     pub(crate) fn bufs_cnt(&self) -> usize {
         self.bufs.len()
     }