take hr and Rust version updates from the rust-series branch
take fifth post in Rust path tracer series from the rust-series branch
revise prospect in Rust path tracer post 4

Writing these posts is a natural process, it is hard to tell exactly
what comes next. I thought I'd briefly discuss units and then write
about the task scheduler, but the units deserve their own post.
use non-breaking space before and after guillemets