Easier interface for TCP ::connect and ::bind.

Prior to this commit, TcpStream::connect and TcpListener::bind took a
single SocketAddr argument. This worked well enough, but the API felt a
little too "low level" for most simple use cases.

A great example is connecting to rust-lang.org on port 80. Rust users would
need to:

  1. resolve the IP address of rust-lang.org using
     io::net::addrinfo::get_host_addresses.

  2. check for errors

  3. if all went well, use the returned IP address and the port number
     to construct a SocketAddr

  4. pass this SocketAddr to TcpStream::connect.

I'm modifying the type signature of TcpStream::connect and
TcpListener::bind so that the API is a little easier to use.

TcpStream::connect now accepts two arguments: a string describing the
host/IP of the host we wish to connect to, and a u16 representing the
remote port number.

Similarly, TcpListener::bind has been modified to take two arguments:
a string describing the local interface address (e.g. "0.0.0.0" or
"127.0.0.1") and a u16 port number.

Here's how to port your Rust code to use the new TcpStream::connect API:

  // old ::connect API
  let addr = SocketAddr{ip: Ipv4Addr{127, 0, 0, 1}, port: 8080};
  let stream = TcpStream::connect(addr).unwrap()

  // new ::connect API (minimal change)
  let addr = SocketAddr{ip: Ipv4Addr{127, 0, 0, 1}, port: 8080};
  let stream = TcpStream::connect(addr.ip.to_str(), addr.port()).unwrap()

  // new ::connect API (more compact)
  let stream = TcpStream::connect("127.0.0.1", 8080).unwrap()

  // new ::connect API (hostname)
  let stream = TcpStream::connect("rust-lang.org", 80)

Similarly, for TcpListener::bind:

  // old ::bind API
  let addr = SocketAddr{ip: Ipv4Addr{0, 0, 0, 0}, port: 8080};
  let mut acceptor = TcpListener::bind(addr).listen();

  // new ::bind API (minimal change)
  let addr = SocketAddr{ip: Ipv4Addr{0, 0, 0, 0}, port: 8080};
  let mut acceptor = TcpListener::bind(addr.ip.to_str(), addr.port()).listen()

  // new ::bind API (more compact)
  let mut acceptor = TcpListener::bind("0.0.0.0", 8080).listen()

[breaking-change]
Try to parse TcpStream::connect 'host' parameter as an IP.

Fall back to get_host_addresses to try a DNS lookup if we can't
parse it as an IP address.
Document a possible way in which connect_timout may change in the future
auto merge of #13919 : thomaslee/rust/thomaslee_proposed_tcpstream_open, r=alexcrichton

Been meaning to try my hand at something like this for a while, and noticed something similar mentioned as part of #13537. The suggestion on the original ticket is to use `TcpStream::open(&str)` to pass in a host + port string, but seems a little cleaner to pass in host and port separately -- so a signature like `TcpStream::open(&str, u16)`.

Also means we can use std::io::net::addrinfo directly instead of using e.g. liburl to parse the host+port pair from a string.

One outstanding issue in this PR that I'm not entirely sure how to address: in open_timeout, the timeout_ms will apply for every A record we find associated with a hostname -- probably not the intended behavior, but I didn't want to waste my time on elaborate alternatives until the general idea was a-OKed. :)

Anyway, perhaps there are other reasons for us to prefer the original proposed syntax, but thought I'd get some thoughts on this. Maybe there are some solid reasons to prefer using liburl to do this stuff.