Consider introducing executor_traits::async_progress_category

[jaredhoberock]

This type would characterize the progress guarantees of the asynchronous operations async_execute() and then_execute() with respect to the caller of these operations.

P0072 provides a good first draft at what these would look like. We might consider tweaking the names slightly:

struct weakly_parallel_progress_tag {};
struct parallel_progress_tag : weakly_parallel_progress_tag {};
struct concurrent_progress_tag : parallel_progress_tag {};

The inheritance relationships also describe the relative strength of these guarantees.

The default progress category would need to be the weakest guarantee. We should make sure that this correctly categorizes the default implementation of executor_traits::async_execute(), which is equivalent to std::async(std::launch::deferred, ...).

Correctly optimizing the implementation of executor_array::then_execute() depends on the ability to introspect this progress guarantee.