feat(http1): Add higher-level HTTP upgrade support to Client and Server (#1563)

- Adds `Body::on_upgrade()` that returns an `OnUpgrade` future.
- Adds `hyper::upgrade` module containing types for dealing with
  upgrades.
- Adds `server::conn::Connection::with_upgrades()` method to enable
  these upgrades when using lower-level API (because of a missing
  `Send` bound on the transport generic).
- Client connections are automatically enabled.
- Optimizes request parsing, to make up for extra work to look for
  upgrade requests.
  - Returns a smaller `DecodedLength` type instead of the fatter
    `Decoder`, which should also allow a couple fewer branches.
  - Removes the `Decode::Ignore` wrapper enum, and instead ignoring
    1xx responses is handled directly in the response parsing code.

Ref #1563 

Closes #1395

src/proto/h1/io.rs

@@ -93,6 +93,12 @@ where
         self.read_buf.as_ref()
     }
 
+    #[cfg(test)]
+    #[cfg(feature = "nightly")]
+    pub(super) fn read_buf_mut(&mut self) -> &mut BytesMut {
+        &mut self.read_buf
+    }
+
     pub fn headers_buf(&mut self) -> &mut Vec<u8> {
         let buf = self.write_buf.headers_mut();
         &mut buf.bytes
@@ -595,7 +601,7 @@ mod tests {
             cached_headers: &mut None,
             req_method: &mut None,
         };
-        assert!(buffered.parse::<::proto::ClientTransaction>(ctx).unwrap().is_not_ready());
+        assert!(buffered.parse::<::proto::h1::ClientTransaction>(ctx).unwrap().is_not_ready());
         assert!(buffered.io.blocked());
     }