Add unwind feature flag

[kstrafe]

Specifying the unwind feature flag removes the catch_unwind from
the task harness. This is especially useful for testing a program with
detached joinhandles.

Motivation

Solving #2699 to make testing easier to perform. Especially system tests where we have no control over spawned and detached tasks. Not all systems are going to be running with catch_unwind on each task. Some want to fail fast.

Solution

Adds a feature unwind to disable the usage of catch_unwind.

[carllerche]

How does this work with the multi-threaded scheduler?

[kstrafe]

@carllerche no idea, I suppose the thread just swallows the panic. This is not intended for use with the multithreaded scheduler tho. So it can be somehow disabled for that.

[carllerche]

If I understand the PR correctly, I don't think the feature flag is additive. feature flags should not change the behavior of code, only add new APIs or change internal runtime details.

I would suggest writing up a proposal considering how unwinding fits with the various schedulers. I added additional thoughts here: #2699 (comment)

I appreciate the PR, there is definitely unexpected complexity. I am going to close this PR. If you want to continue the effort, I would recommend opening an issue containing a proposal addressing the edge cases. You can also ping people in #tokio-dev on Discord (https://discord.gg/tokio).