This fixes the regression where process names weren't appearing for
Darwin probes. Makes testing easier.
Also, changes the process walker to operate on value types. There's no
performance advantage to using reference types for something of this
size, and there appeared to be a data race in the Darwin port that
caused nodes to gain and lose process names over time.
Also, restructures how to enable docker scraping. Default false when run
manually, and enabled via --probe.docker true in the scope script.
- Move pidtree to its own module and disaggregate it into tree, walker and reporter.
- Extend testing for probe/process
- Extend process metadata; add command line & # threads.
