Add num_cpus, threadpool to stdx

[brson]

See https://github.com/brson/stdx/blob/master/CONTRIBUTING.md for details on how to add a crate.

[brson]

For an example, create a threadpool using the threadpool crate, with the number of cpus on the system. Since that requires threadpool, might as well add threadpool to stdx as well.

Duplicate the example so both num_cpus and threadpool have the same example, similar to flate2 / tar.