Reduce the complexity around rust_sched_launcher

[brson]

In order to support the awkward requirement of running Rust tasks on the main thread, scheduler thread management duties are divided between a bunch of tiny classes: rust_sched_driver, rust_sched_launcher, rust_sched_launcher_factory, and their subclasses. I suspect this can be simplified, though it's been a long time since I looked at it.

Also, the names, long though they are, are confusingly inaccurate. The rust_sched_launcher and factory are actually for launching scheduler threads, not schedulers as the name would lead you to believe.

[nikomatsakis]

Not critical for 0.6; removing milestone

[Aatch]

This isn't relevant any more with the new scheduler and runtime.