If you move a coroutine, all references to the coroutine from
inside of it become invalid, including e.g. yielders. Therefore,
make coroutines immovable to prevent weird bugs...
but also, make the guts of coroutine context a bit more move
friendly (like, do not change current status after context switch
and always do it outside), in case a solution is found in the
future.
Daniel Kolesa
493f31fabf