Added ToSocketAddr trait

This commit adds ToSocketAddr trait to std::io::net::ip module. This
trait is used for generic conversion from different types (strings,
(string, u16) tuples, etc.) into a SocketAddr instance. It supports
multiple output SocketAddresses when it is appropriate (e.g. DNS name
resolution).

This trait is going to be used by TcpStream, TcpListener and UdpSocket
structures.
Switched io::net::tcp to use ToSocketAddr

TcpListener and TcpStream are converted to use ToSocketAddr trait in
their constructor methods.

[breaking-change]
Migrated io::net::udp over to ToSocketAddr

UdpSocket constructor methods now use ToSocketAddr trait instead of
SocketAddr.

[breaking-change]
Fixed other tests to pass make check
Fixed tidy errors
Added more documentation on ToSocketAddr trait
Fixed not compiling code in docstring
auto merge of #18462 : netvl/rust/to-socket-addr, r=alexcrichton

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.
Runtime removal: add private sys, sys_common modules

These modules will house the code that used to be part of the runtime system
in libnative. The `sys_common` module contains a few low-level but
cross-platform details. The `sys` module is set up using `#[cfg()]` to
include either a unix or windows implementation of a common API
surface. This API surface is *not* exported directly in `libstd`, but is
instead used to bulid `std::os` and `std::io`.

Ultimately, the low-level details in `sys` will be exposed in a
controlled way through a separate platform-specific surface, but that
setup is not part of this patch.
Remove signal handling.

Since signal handling was only implemented through librustuv, which is
now gone, there's no reason to even provide the API.

[breaking-change]
Runtime removal: refactor fs

This moves the filesystem implementation from libnative into the new
`sys` modules, refactoring along the way and hooking into `std::io::fs`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor pipes and networking

This patch continues the runtime removal by moving pipe and
networking-related code into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor helper threads

This patch continues the runtime removal by moving
libnative::io::helper_thread into sys::helper_signal and
sys_common::helper_thread

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor process

This patch continues the runtime removal by moving and refactoring the
process implementation into the new `sys` module.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor timer

This patch continues runtime removal by moving out timer-related code
into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: refactor tty

This patch continues runtime removal by moving the tty implementations
into `sys`.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.
Runtime removal: fully remove rtio

This patch cleans up the remnants of the runtime IO interface.

Because this eliminates APIs in `libnative` and `librustrt`, it is a:

[breaking-change]

This functionality is likely to be available publicly, in some form,
from `std` in the future.