async-panic

The tracking issue for this feature is: #164106.

Part of project goal async statemachine optimisation.

When set to true (default), async blocks and functions will generate a statemachine that will emit a panic when the statemachine is polled after completion.

When set to false, a poll after completion will yield a Poll::Pending value. This makes the statemachine simpler and saves on binary size. The downside is that buggy code may poll a future that will never yield Ready(_) anymore which can lead to getting stuck in infinite loops or general inefficiencies.