high-level tcp bindings for std

### quick summary

This pull req includes high-level TCP/IP bindings for the rust stdlib (both server and client API), under the `std::net::tcp` module. I also did some reshuffling of the IP stuff and pushed it into the `net::ip` module.

### only slightly more in-depth

An example demonstrating the TCP client/request API can be found [here](https://github.com/olsonjeffery/rust/blob/hl_tcp/src/libstd/net_tcp.rs#L1291).

Interestingly, because of API-style friction between libuv and rust, I found it neccesary to expose two separate UIs to the TCP server API. Quickly, with examples, they are:

* the `net::tcp::listen_for_conn` entry point, an example of which is [here](https://github.com/olsonjeffery/rust/blob/hl_tcp/src/libstd/net_tcp.rs#L1164). This call is blocking for the lifetime of the TCP server connection and uses a callback running on the libuv loop (!).
* and the `net::tcp::new_listener` entry point, an example of which can be found [here](https://github.com/olsonjeffery/rust/blob/hl_tcp/src/libstd/net_tcp.rs#L1239). This bit of API, on successful setup, returns a (vaguely) `comm::port`-like object that uses can peek/recv on (using custom API), as needed, to get new connections.

The `new_listener` API varies, subtly, from the `listen_for_conn` entry point because it *accepts new connections immediately* (with the overhead that this entails), while `listen_for_conn` gives a user the chance to manually `accept` a connection, or just drop it, in line with the default behavior of libuv.

To more fully articulate this, I'm just going to paste my commit msg from `95f0b90`:

- we now have two interfaces for the TCP/IP server/listener workflow,
based on different user approaches surrounding how to deal with the
flow of accept a new tcp connection:

1. the "original" API closely mimics the low-level libuv API, in that we
have an on_connect_cb that the user provides *that is ran on the libuv
thread*. In this callback, the user can accept() a connection, turning it
into a tcp_socket.. of course, before accepting, they have the option
of passing it to a new task, provided they *make the cb block until
the accept is done* .. this is because, in libuv, you have to do the
uv_accept call in the span of that on_connect_cb callback that gets fired
when a new connection comes in. thems the breaks..

I wanted to just get rid of this API, because the general proposition of
users always running code on the libuv thread sounds like an invitation
for many future headaches. the API restriction to have to choose to
immediately accept a connection (and allow the user to block libuv as
needed) isn't too bad for power users who could conceive of circumstances
where they would drop an incoming TCP connection and know what they're
doing, in general.

but as a general API, I thought this was a bit cumbersome, so I ended up
devising..

2. an API that is initiated with a call to `net::tcp::new_listener()` ..
has a similar signature to `net::tcp::listen()`, except that is just
returns an object that sort of behaves like a `comm::port`. Users can
block on the `tcp_conn_port` to receive new connections, either in the
current task or in a new task, depending on which API route they take
(`net::tcp::conn_recv` or `net::tcp::conn_recv_spawn` respectively).. there
is also a `net::tcp::conn_peek` function that will do a peek on the
underlying port to see if there are pending connections.

The main difference, with this API, is that the low-level libuv glue is
going to *accept every connection attempt*, along with the overhead that
that brings. But, this is a much more hassle-free API for 95% of use
cases and will probably be the one that most users will want to reach for.

### what's needed to fill this out

1. IPv6! I stubbed out the data structure, but parsing IPv6 addr strings is non-trivial, so I'm not sure how to tackle this from rust, just yet. If anyone wants to pitch in, that'd be swell.
2. The other bits of the tcp API. I only added connection-establishment for clients/server and read/write over TCP streams.. so stuff like setting keep-alive, etc needs to be added.