ToSocketAddr trait and unification of network structures constructor methods

[netvl]

This is a follow-up to RFC PR #173. I was told there that changes like this don't need to go through the RFC process, so I'm submitting this directly.

This PR introduces ToSocketAddr trait as defined in said RFC. This trait defines a conversion from different types like &str, (&str, u16) or even SocketAddr to SocketAddr. Then this trait is used in all constructor methods for TcpStream, TcpListener and UdpSocket.

This unifies these constructor methods - previously they were using different types of input parameters (TCP ones used (&str, u16) pair while UDP ones used SocketAddr), which is not consistent by itself and sometimes inconvenient - for example, when the address initially is available as SocketAddr, you still need to convert it to string to pass it to e.g. TcpStream. This is very prominently demonstrated by the unit tests for TCP functionality. This PR makes working with network objects much like with Path, which also uses similar trait to be able to be constructed from &[u8], Vec<u8> and other Paths.

This is a breaking change. If constant literals were used before, like this:

TcpStream::connect("localhost", 12345)

then the nicest fix is to change it to this:

TcpStream::connect("localhost:12345")

If variables were used before, like this:

TcpStream::connect(some_address, some_port)

then the arguments should be wrapped in another set of parentheses:

TcpStream::connect((some_address, some_port))

UdpSocket usages won't break because its constructor method accepted SocketAddr which implements ToSocketAddr, so bind() calls:

UdpSocket::bind(some_socket_addr)

will continue working as before.

I haven't changed UdpStream constructor because it is deprecated anyway.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @aturon (or someone else) soon.

[huonw]

cc @alexcrichton

[seanmonstar]

Excellent! It really bothered me that in hyper I was allocating a String from a SocketAddr just to have the connect method create a new one.

[alexcrichton]

There's actually a FromStr implementation for SocketAddr which this should probably use instead

[alexcrichton]

Hm nevermind, I see what's going on here

[alexcrichton]

This looks great to me, thanks @netvl! Just wanna run it by @aturon as well to make sure I didn't miss anything conventions-wise.

[netvl]

I also want to add a little documentation on ToSocketAddr trait first, and probably tweak it on methods which use ToSocketAddr, if you don't mind.

[netvl]

I've added some docs on ToSocketAddr, so I think it is ready. Let me know if I there is something else I need to fix :)

[aturon]

This looks fantastic! I'm happy for this to land (needs a rebase, though.)

[netvl]

Rebased but still need to run tests to check that rebase was correct.

[alexcrichton]

Thanks again for this @netvl!