Make `num_domains` argument to `Task.setup_pool` optional

[dra27]

The OCaml 5.0 includes Domain.recommended_domain_count which seems to be a reasonable default value for Task.setup_pool. Making the argument optional is code-compatible with existing code.

This would also create a mechanism within Task to use Domain.recommended_domain_count without incurring the "unstable" alert on the Domain module.

[kayceesrk]

This sounds like a good idea.

Separately, I am also wondering whether to make the pool an optional argument. The current pool can be fetched from the context now that we have an effect handler in domainslib.