tracing, tracing-futures: instrument futures with Spans in Drop

[zohnannor]

Resolves #2541

Motivation

See #2541

Solution

Wrap Instrumented::inner (both in tracing and tracing-futures) into Option. Pin::set it to None in the PinnedDrop impl to drop the future in the context of a Span that we enter in the said impl.

This implementation uses Option::unwrap_unchecked in places where we need to extract inner future, as the only time we set it to None is in Drop, or in a method which takes Instrumented by value, guaranteeing that there is no case for UB.

Alternative was to use ManuallyDrop, but I don't think we can Future::poll Pin<&mut ManuallyDrop<T>> easily?

[zohnannor]

Closed in favor of #2562