- 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 #1563Closes#1395
