Newrt file io

This PR includes the addition of the essential CRUD functionality exposed as a part of the `uv_fs_*` api. There's a lot more to be done, but the essential abstractions are in place and can be easily expaneded.

A summary:

* `rt::io::file::FileStream` is fleshed out and behaves as a *non-positional* file stream (that is, it has a cursor that can be viewed/changed via `tell` and `seek`
* The underlying abstraction in `RtioFileStream` exposes pairs of `read(), write()` and `pread(), pwrite()`. The latter two take explicit `offset` params and don't respect the current cursor location in a file afaik. They both use the same underlying libuv impl
* Because libuv explicitly does *not* support `seek`/`tell` operations, these are impl'd in `UvFileStream` by using `lseek(2)` on the raw file descriptor.
* I did my best to flesh out and adhere to the stubbing that was already present in `rt::io::file` and the tests should back that up. There may be things missing.
* All of the work to test `seek`/`tell` is done in `rt::io::file`, even though the actual impl is down in `rt::uv::uvio`.

There are a few polish things I need to do still (the foremost that I know of is undefined behavior when seek'ing beyond the file's boundary).

After this lands, I want to move on to mapping more of the `uv_fs_*` API (especially `uv_fs_stat`). Also a few people have mentioned interest in `uv_pipe_t` support. I'm open to suggestions.