making simple-process-stats optional

[jondot]

Hi,
There is a dependency issue, which causes unresolvable Cargo conflicts on macOS caused by a transitive dependency of simple-process-stats:

• see upgrade memchr dependency heim-rs/darwin-libproc#9 on the core dependency issue, and the fact that darwin-libproc is not maintained actively to fix it
• sidekiq-rs needs robotty/simple-process-stats which depends on darwin-libproc, which depends on
  a too old memchr version which many libraries, one example: sqlx needs. because libproc is native and not pure rust, cargo cannot resolve it.

since simple_process_stats use by sidekiq-rs is minimal only for getting RSS, its better to replace it with something that has more velocity on updating dependencies but that is also lightweight and cross platform (I didn't find one yet).

in the meanwhile as a workaround, I put the dependency under an optional flag -- enabled by a feature. this feature is by-default turned on so no one will notice.

in our codebase, we can disable this feature, which disables simple-process-stats, and our build passes nicely.

[film42]

Looks great. Going to add a comment at the cfg to state why and under what conditions this can be removed. Thank you!

[film42]

Released as 0.8.2.