Runner support for MacOS

[mhofman]

The runner relies on Linux's procfs to handle some process specific operations:

• Required functionality: List children of a given process, and cmdline
• Optional: list process utilization and uptime

The provider is fairly well abstracted but we should have an implementation for MacOS, at least for the required functionality.

[mhofman]

Potential resources:

• https://github.com/yibn2008/find-process/blob/master/lib/find_process.js
• https://stackoverflow.com/questions/11546765/how-to-get-the-pid-of-a-process-by-giving-the-process-name-in-mac-os-x

[mhofman]

Implementing #54 would intrinsically remove the dependency on agoric-cli, which would remove the required functionality to find sub processes of our spawned children. In that case, we would be left with the optional CPU/Memory metrics.