Adapt executor implementation to P0443R2

[hkaiser]

This involves a couple of steps. The (currently still evolving) document can be found here: https://github.com/executors/issaquah_2016/blob/R2/wording.md

• clean up existing executor categories and corresponding traits (P0443 r2 #2693)
• rename <op>_bulk_execute to bulk_<op>_execute (P0443 r2 #2693)
• implement BaseExecutor requirement on all existing executors
• implement traits has_<op>_member<T> (P0443 r2 #2693)
• implement traits has_<op>_free_function<T>
• support free functions when invoking executors (use traits above to distinguish)
• implement traits reporting on customization points can_<op><T>
• implement task_block overload taking executor (as an alternative to the existing overload taking an execution policy)
• add executor_execution_mapping_guarantee
• add executor_execute_blocking_guarantee
• add executor_bulk_forward_progress_guarantee
• implement polymorphic executor wrappers
• implement default one_way_executor, two_way_executor, and never_blocking_one_way_executor
• implement non-variadics interface requirements of P0443R2 on top of our variadic interfaces

[msimberg]

@hkaiser we can probably close this now? I think it's unlikely that we'll go back and fill in the gaps for an old proposal. We can track the current progress in #5045.