Towards removing std::io

This implements a number of the baby steps needed to start eliminating everything inside of `std::io`. It turns out that there are a *lot* of users of that module, so I'm going to try to tackle them separately instead of bringing down the whole system all at once.

This pull implements a large amount of unimplemented functionality inside of `std::rt::io` including:

* Native file I/O (file descriptors, *FILE)
* Native stdio (through the native file descriptors)
* Native processes (extracted from `std::run`)

I also found that there are a number of users of `std::io` which desire to read an input line-by-line, so I added an implementation of `read_until` and `read_line` to `BufferedReader`.

With all of these changes in place, I started to axe various usages of `std::io`. There's a lot of one-off uses here-and-there, but the major use-case remaining that doesn't have a fantastic solution is `extra::json`. I ran into a few compiler bugs when attempting to remove that, so I figured I'd come back to it later instead. 

There is one fairly major change in this pull, and it's moving from native stdio to uv stdio via `print` and `println`. Unfortunately logging still goes through native I/O (via `dumb_println`). This is going to need some thinking, because I still want the goal of logging/printing to be 0 allocations, and this is not possible if `io::stdio::stderr()` is called on each log message. Instead I think that this may need to be cached as the `logger` field inside the `Task` struct, but that will require a little more workings to get right (this is also a similar problem for print/println, do we cache `stdout()` to not have to re-create it every time?).