Add peek to TcpStream.

[nickgian]

Our codebase is using the peek functions that Turmoil does not model. It seemed easy to add so here's an attempt to add them for TCP.

• poll_peek
• peek

[mcches]

I'll be out for a couple of days so my review will be delayed until later in the week. Adding @LucioFranco if you have availability.

I haven't looked yet, but we'll want to ensure #184 fits into whatever is added here.