Introduce net.cgrand.xforms/parallel

[opqdonut]

I've had this utility lying around for years, and finally decided to
contribute it somewhere. I hope it's a good fit for xforms.

Thanks for the great test suite! I had to add proper handling of
reduced values to make the trial pass.

A slightly more useful version of parallel would let one choose a
chunk size instead of parallelizing element-by-element. That would
probably be a bit more work, so I didn't include it in this PR but
might be interested in implementing it later.

[opqdonut]

ping @cgrand