add support for camino Utf8Path and Utf8PathBuf

[sunshowers]

Add support behind a non-default "camino" feature.

Closes #90.

(One question I have is whether the feature should be called camino or camino1. I personally prefer baking in the semver major version into the feature name but opinions vary.)

[sunshowers]

Should I also change the github config to run on PRs?

[oconnor663]

Should I also change the github config to run on PRs?

Oh that's a good idea, yes please.

[sunshowers]

Updated! Sorry about the delay, this looks good to go now (hopefully tests pass)

[oconnor663]

(Apologies, I've had a busy time at work and built up a backlog of reviews on personal projects. But I haven't forgotten about this!)

[gibfahn]

This is maybe obvious, but until we have this feature, the workaround is to use .as_str() on the path, e.g.:

let echo = Utf8Path::new("/bin/echo");

cmd!(echo.as_str(), "hi").run().unwrap();