New I/O API, scheduler improvements

r?

This pull request is a grab bag of work on the new scheduler.

The most important commit here is where I [outline](https://github.com/brson/rust/blob/io/src/libcore/rt/io/mod.rs) a fairly complete I/O API, based on `Reader` and `Writer` types, as in the current `core::io` module. I've organized this version into a number of modules with declarations for Files, TCP, UDP, Unix sockets, blocking/non-blocking implementations, memory buffers, compression adapters. I'm trying to get this into shape to present on the mailing list.

This branch also wires up `spawn` to the new scheduler, and simplifies the core scheduler operations.