Scheduler + I/O

r? @graydon, @nikomatsakis, @pcwalton, or @catamorphism

Sorry this is so huge, but it's been accumulating for about a month. There's lots of stuff here, mostly oriented toward enabling multithreaded scheduling and improving compatibility between the old and new runtimes. Adds task pinning so that we can create the 'platform thread' in servo.

[Here](https://github.com/brson/rust/blob/e1555f9b5628af2b6c6ed344cad621399cb7684d/src/libstd/rt/mod.rs#L201) is the current runtime setup code.

About half of this has already been reviewed.