Default to using multi-threading

[kesyog]

For the sake of ergonomics and performance, IMO the default number of threads should be the number of CPU's. In my experience, the cases where single-threaded behavior is required are the exception, not the rule, and users can explicitly opt in for single-threaded behavior if needed.

This would indirectly solve a different issue: the only way to automatically peg the number of threads to the number of CPU's is to set --worker 0 --threads 0 even though the worker argument is listed as deprecated.

[kesyog]

Hey @lotabout, I'm curious what you think about this issue and the linked PR. I think it's a great ergonomics improvement, as 99% (if not 100%) of my use cases for rargs/xargs are trivially parallel.