Implement automatic thread number resolving for RustKZG

[zilm13]

See variable NUMBER_OF_THREADS in RustKZG.
It should be adjusted automatically to utilize available threads and to make performance of Rust library on par with CKZG.

[zilm13]

Looks like it's possible to make library work the same way like ckzg, so we don't need to pass it into constructor and it will work with parallel() the same way like ckzg does. So we shouldn't hurry, probably the library will be updated and it will work just out of the box.