More scheduler and I/O work

r?

There are a lot of commits here, but not all that much substance. Mostly just refactoring.

I started sketching out the beginnings of a very simple I/O API in `core::rt::io` that represents I/O streams as a single `Stream` trait instead of `Reader` / `Writer` pairs. This seems to be the more common pattern (at least this is how the .NET BCL does it) and it seems to me that separate readers and writers would make duplex streams very awkward. Regardless, I don't intend to go very far down the I/O API design road without some mailing list discussion.

I've also started on the uv bindings for file I/O but haven't gotten very far.

Also hooked up the new scheduler to `rust_start` and the compiletest driver. 70% of run-pass test cases already pass, but I wouldn't read too much into that.

I also split the direct, low-level uv bindings in two so that the scheduler can have its own set, leaving `std::net` on its own.