Join to a new thread on exhaustion

[pepyakin]

As an idea for fallback: on unsupported platforms spawn a new thread and join the current thread to the newly created. This is portable but works only with std though.

[alexcrichton]

Perhaps! I think the only downside is that TLS wouldn't work, but perhaps that just means this could be opt-in?

[nagisa]

An additional concern is that we would be limited to executing callbacks that are Send only, which would introduce an unnecessary constraint on platforms which do support switching.

We currently support a fair number of targets and it is not unfeasible to add more, so I think this issue is and will become less relevant over time.