Rust: adjust number of threads

[mike-barber]

Description

The number of physical cores is not reliably detected on many of the benchmark machines, and it's always a fun problem with containers and virtualization. This little PR makes some small adjustments to the runs to deal with this sensibly:

• changed physical cores count to simple (logical_cores / 2), similar to other solutions
• added run on 4 threads (if possible) for comparability with @GordonBGood updates
• removed some of the older "striped" solutions from default run
  • less noise in reports from the earlier algorithms
  • less time for the overall run

Contributing requirements

• [x] I read the contribution guidelines in CONTRIBUTING.md.
• [x] I placed my solution in the correct solution folder.
• [x] I added a README.md with the right badge(s).
• [x] I added a Dockerfile that builds and runs my solution.
• [x] I selected drag-race as the target branch.
• [x] All code herein is licensed compatible with BSD-3.

[rbergen]

@mike-barber Thank you for submitting this update!