Teach rustc about spawning and tasks

[graydon]

This is a long-ish piece of work, and not essential for bootstrapping, but rustc doesn't know anything about the tasking system yet. Needs to eventually.

[graydon]

brson did a fair amount of this but it is not 100% live yet.

[eholk]

Spawning tasks is working now, although we don't have threading yet so it's not as useful yet. I'll go ahead and close this and open another issue for running tasks on actual threads.