Provide "peek" APIs to `std::net` sockets

[APTy]

Summary

Provide "peek" APIs to std::net sockets, for example:

• UdpSocket.peek()
• UdpSocket.peek_from()
• TcpStream.peek()

These methods enable socket reads without side-effects. That is, repeated calls to peek() return identical data. This can be accomplished by providing the POSIX flag MSG_PEEK to the underlying socket read operations.

Motivation

This is valuable in cases where a user wishes to validate if new data exists in the socket, without consuming that data completely. For example, a user may wish to use peek() as a method of polling the socket to find out if new messages are available (or to determine the amount of data available), before deferring to another function to do the actual work of reading and parsing the socket's data.

[cseale]

This sounds like a good candidate for a first PR, would you think? If so I would like to take this on, with maybe a bit of help?

[APTy]

Thanks for offering! I am planning to submit a PR for a part 1 (UdpSocket implementation), and needed an issue to reference for the feature flag. Feel free to tackle TcpStream in parallel :)

[cseale]

Cool, I give that a go!

[cseale]

Looking at your branch, it might be an aggressive commit for me for my first contribution, unless perhaps you could provide me with a bit of detail on your intended implementation or somewhere maybe I could go for reference?

[APTy]

That's okay, I went ahead and added the TcpStream implementation, since it was pretty closely related to the Socket updates I was making. Thanks anyway!

[alexcrichton]

Reopening as the tracking issue for this feature