Missing piece: TLS support

[njsmith]

It should be easy and ergonomic to use TLS with trio. This needs to be in the core.

My general idea is that trio.socket only exposes raw sockets, not fake-sockets-that-have-ssl-wrapped-around-them, and then we have a Stream adaptor that applies TLS. Initially using ssl.SSLObject and the BIO interfaces, but we should also keep a close eye on the plans for a new set of TLS APIs in the 3.7 time-frame (unfortunately now is kind of the worst time to be defining a TLS API!): Lukasa/peps#1

Anyway, I like this general design, but there are questions about how to make it ergonomic. Getting a socket + TLS is a pretty fundamental thing; we don't want it to feel cumbersome. And you might want to do things like call getsockopt or getpeername on your socket, which is tricky if you just called something like create_connection that handed you back a wrapped stream that doesn't have those methods! (Or worse, might hand back either a stream or a socket depending on the arguments you pass.) OTOH it would be nice if we can also make it pleasant to work with more complicated things like SNI callbacks etc. -- in jongleur I found curio's helper stuff pretty useless and immediately moved to separate socket and wrapped-socket objects anyway.

The thing where both for clients both the socket connect and the TLS handshake need to know the remote hostname is also a challenge for layering.

#8 is closely related.

[njsmith]

Added in #107