Auto merge of #2699 - RalfJung:schedule-refactor, r=RalfJung

refactor scheduler

Refactors the scheduler to use something akin to a generator -- a callback that will be invoked when the stack of a thread is empty, which has the chance to push a new stack frame or do other things and then indicates whether this thread is done, or should be scheduled again. (Unfortunately I think we [cannot use actual generators](https://rust-lang.zulipchat.com/#narrow/stream/213817-t-lang/topic/Generators.20that.20borrow.20on.20each.20resume.3F) here.) The interpreter loop is now a proper infinite loop, the only way to leave it is for some kind of interrupt to be triggered (represented as `InterpError`) -- unifying how we handle 'exit when calling `process::exit`' and 'exit when main thread quits'.

The last commit implements an alternative approach to https://github.com/rust-lang/miri/pull/2660 using this new structure. Fixes https://github.com/rust-lang/miri/issues/2629.

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