while still preserving the guarantee about async() being assigned a unit
of concurrency (or immediately running the task), this change:
* retains the error from calling getCpuCount()
* spawns all threads in detached mode, using WaitGroup to join them
* treats all workers the same regardless of whether they are processing
concurrent or async tasks. one thread pool does all the work, while
respecting async and concurrent limits.
This fixes package fetching on Windows.
Previously, `Async/GroupClosure` allocations were only aligned for the
closure struct type, which resulted in panics when `context_alignment`
(or `result_alignment` for that matter) had a greater alignment.
