Add SteamNetworkingSockets

[casid]

First off, thank you for this library @code-disaster!

For my game on Steam I have used SteamNetworking. Over time, I have received frequent complaints from players about unstable P2P matchmaking, connection drops and lag. Especially Chinese players seem to have lots of issues on the 'legacy' networking.

I took the existing fork from Tyler as base, with the following changes:

• Use Connection, Socket and SteamId values over raw ints or longs
• Use ConnectionState enum over raw ints in callback
• Provide all available SendFlags bitflags as constants (here int seems to be the more natural choice)
• receiveMessageOnConnection does not allocate
• Add a small utility method enableSymmetricConnect, because it simplified P2P handshakes a lot
• Removed all poll group methods because I had no need for them and no way to test them

This is already running in production and so far I have only received positive feedback from players.

I'd really love to see this integrated, and I'm more than happy to change or refine anything needed to make that happen.