allow_duplicates should work for async code too

I want to make my tests async so I can easily parallelize them for speed. I also want to use allow_duplicates!. Unfortunately this doesn't work as it uses a non-async helper function, and if I were to have that function return a future then the future wouldn't support duplicates as the code would not be running from within the context of allow_duplicates! anymore.

What I'd like is an allow_duplicates_async! version that acts like an async {} block except allowing duplicates for code running in the future. This could work by returning a custom future that sets up the duplicates infrastructure on each poll (thus still allowing for the use of thread-local state in the implementation).

Start by locating the implementations of allow_duplicates! and async {} and the future behavior described in the issue. Determine how duplicate tracking is initialized across future polls; done means an async counterpart permits duplicates for code running in the future without losing the required context.