core: Add rt::context for figuring out what runtime services are available

Conflicts:
	src/libcore/rt/sched/mod.rs
core: Don't use managed boxes in TaskBuilder
core: Wire up `spawn` to the new scheduler

It will check which scheduler it is running under and create the
correct type of task as appropriate. Most options aren't supported
but basic spawning works.
core: More tweaks to the thread-local scheduler interface
wip