ChanHandle shouldn't be Clone

Also wake after calling .progress(), which is when we will
have new incoming data.

Otherwise we can spin forever on unhandled packets (such as stderr)

Runner should be a reference

Take is_client argument for Conn::new()

Most code is commented out, won't work

Implements RequestUsername as a trial. Not yet handled by callers.

Async behaviour calls are temporarily commented out. Compiles but
doesn't work correctly.

Everything now builds on stable. Hopefully nightly doesn't break
anything (that's what crater's for, right?)

Uses ~120 bytes more stack. Can be replaced once PR is merged
https://github.com/dalek-cryptography/curve25519-dalek/pull/556

RSA had a few changes in 0.8->0.9 that needed handling

ssh-key's algorithm name is no longer 'static.

The server should expect a banner line straight away, avoids dangling
connections from port scanners etc