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

examples/README.md

@@ -16,4 +16,6 @@ Run examples with `cargo run --example example_name`.
 
 * [`send_file`](send_file.rs) - A server that sends back content of files using tokio_fs to read the files asynchronously.
 
+* [`upgrades`](upgrades.rs) - A server and client demonstrating how to do HTTP upgrades (such as WebSockets or `CONNECT` tunneling).
+
 * [`web_api`](web_api.rs) - A server consisting in a service that returns incoming POST request's content in the response in uppercase and a service that call that call the first service and includes the first service response in its own response.