std::rt improvements

Large topics:

* Implemented `rt::io::net::unix`. We've got an implementation backed by "named pipes" for windows for free from libuv, so I'm not sure if these should be `cfg(unix)` or whether they'd be better placed in `rt::io::pipe` (which is currently kinda useless), or to leave in `unix`. Regardless, we probably shouldn't deny windows of functionality which it certainly has.
* Fully implemented `net::addrinfo`, or at least fully implemented in the sense of making the best attempt to wrap libuv's `getaddrinfo` api
* Moved standard I/O to a libuv TTY instead of just a plain old file descriptor. I found that this interacted better when closing stdin, and it has the added bonus of getting things like terminal dimentions (someone should make a progress bar now!)
* Migrate to `~Trait` instead of a typedef'd object where possible. There are only two more types which are blocked on this, and those are traits which have a method which takes by-value self (there's an open issue on this)
* Drop `rt::io::support::PathLike` in favor of just `ToCStr`. We recently had a lot of Path work done, but it still wasn't getting passed down to libuv (there was an intermediate string conversion), and this allows true paths to work all the way down to libuv (and anything else that can become a C string).