ToSocketAddr trait and unification of network structures constructor methods

This is a follow-up to [RFC PR #173](https://github.com/rust-lang/rfcs/pull/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 `Path`s.

This is a breaking change. If constant literals were used before, like this:
```rust
TcpStream::connect("localhost", 12345)
```
then the nicest fix is to change it to this:
```rust
TcpStream::connect("localhost:12345")
```

If variables were used before, like this:
```rust
TcpStream::connect(some_address, some_port)
```
then the arguments should be wrapped in another set of parentheses:
```rust
TcpStream::connect((some_address, some_port))
```

`UdpSocket` usages won't break because its constructor method accepted `SocketAddr` which implements `ToSocketAddr`, so `bind()` calls:
```rust
UdpSocket::bind(some_socket_addr)
```
will continue working as before.

I haven't changed `UdpStream` constructor because it is deprecated anyway.

Hm nevermind, I see what's going on here