Add command/task runner

[calebcartwright]

I'm leaning towards just for this but still considering cargo-make

[calebcartwright]

After some thinking/working, I no longer view this as necessary. The vast majority of activities (test, build, run, format, lint, etc.) are all already available directly via a cargo subcommand without requiring any args/options.

The only activity that necessitates a longer command is for code coverage. However, as there are some tooling limitations around code coverage (limited number of tools, with limited OS support, with limited support for stable Rust) I expect this will primarily be a CI activity, and not one run regularly as part of the inner dev loop.

I'd rather add instructions to the Contributing docs for folks developing on Linux that want to generate coverage locally as opposed to introducing something like just or cargo-make solely for running coverage.

Closing this accordingly