Draft: Support for rustls

This is based on @janimo's approach in !74 (closed), but diverges in a few important ways.

1. It assumes that something like !251 (merged) will merge, so that we can have separate implementations for native_tls and rustls compiled at the same time.

2. It assumes that we can implement this for the futures::io traits only with no real penalty.

3. It uses the x509-signature crate to work around the pickiness of the webpki crate. If webpki eventually solves their bug 219, we can remove a lot of that workaround.

This work isn't done yet, for several reasons.

• It needs documentation and testing and cleanup.

• Once something like !251 (merged) is mered, this code needs to get integrated with the tokio and async-tls runtimes and exposed to the user.

In its final form, this will resolve #86 (closed).

Thanks to @janimo and @trinity-1686a for their help with this issue.