Spawned tasks are executed out of order

[richarddd]

Since futures are being removed using swap, it can mess up the order of how tasks are spawned: https://github.com/DelSkayn/rquickjs/blob/20d3008821c437b9cf94877a1f29e1b9e1c895d6/core/src/runtime/spawner.rs#L80 I also noticed that ordering can also be wrong if we don't sweep for all the completed futures in a poll and remove them.

This bug is a bit hard to reproduce in a simple use case since it depends on what the futures you are spawning are doing. Consider this simple example: The first task that completes will swap place with the last task, breaking the ordering of the next poll round.

This https://github.com/DelSkayn/rquickjs/pull/201 fixes the issue by looping over all tasks and then removing them from the list. List doesn't need to be atomic since we always push at the end.

[DelSkayn]

Should be fixed now that #201 is merged.