Automatically select ValueHint for Paths when deriving Parser

[jcgruenhage]

Currently, arguments require explicity setting a ValueHint to get files completed. If the argument is a PathBuf or Path though, it's pretty obvious that what we're dealing with is some sort of a path, and defaulting to ValueHint::AnyPath would therefore be the more logical choice.

I'm not sure whether this change is worth it though, since it's probably only applicable to that one type, and even for that type, there are multiple more specific variants which one might choose instead, which is why I'm posting this as a discussion instead of posting it as a feature request issue. Any opinions?

[epage]

The biggest annoyance is detecting types. We just get the item path as written down. We have to then handle std::path::PathBuf vs path::PathBuf vs PathBuf vs .... We've been wanting to bring typed support to the builder api (the core of the derive api) and see if we can automatically handle hints there. That would also make it so we can tie into the type system and more easily detect cases like this.

See #2683

[jcgruenhage]

Oof, I hadn't even thought of that aspect. Sounds like it's even more work than I anticipated, and the gain would still be small. Yay.