Use Box<Path> in lieu of PathBuf for immutable structs

[charliermarsh]

Summary

I don't know if I actually want to commit this, but I did it on the plane last time and just polished it off (got it to compile) while waiting to board.

[zanieb]

I read the title and thought "classic Charlie on the plane PR"

[BurntSushi]

It looks like there's no measurable change in perf, but I still feel like this is something we should do for "good sense" reasons. This also eliminates the possibility of any PathBuf being replaced here containing "extra" capacity (which would be a kind of space leak I guess). But I don't think any of our metrics really track that, so it's hard to know whether there is an actual improvement or not.

One interesting thing to consider would be a "small path" optimization. I'm not sure a crate for that exists. (A quick search didn't turn anything up.) It might be a little trickier to build than a small string optimization.