Add convenience method for polling Task<>

[TheRawMeatball]

Objective

Checking if a task is completed and retrieving the value is too convoluted

Solution

Add a simple convenience method

[hymm]

Is this an alternative to #4102?

If it is what are the advantages/disadvantages vs that PR?

[TheRawMeatball]

Lower overhead, as this doesn't require adding an extra channel, instead using what's already built into async_executor::Task

[DJMcNab]

But if checking it is more expensive, then I would think that cost would dwarf the extra overhead of the channel.

That is, I can't just let the claim of 'lower overhead' meaning better go by without confirmation - Task::poll isn't a trivial function, and has more inherent overhead than a one-shot channel - because it must behave properly when an actual proper future is awaiting it. Maybe counting moves of the value changes that equation, but I don't find it as obviously clear cut as you.

[alice-i-cecile]

Added the Controversial tag. I'd like to see perf testing here. The API is really simple and appealing, but I don't want that to turn into a footgun.

[hymm]

I added perf numbers here #4102 (comment).