Add aliases to std::path::Path via extension trait

[Emerentius]

This PR is based on #25.

Path has some nice methods which are just shortcuts for free functions in std::fs. path.some_fn() => std::fs::some_fn(path). Having to translate such calls to free function calls to benefit from this crate is bothersome.

This PR adds aliases that work the same way except they call fs_err::some_fn. Because inherent methods take precedence over trait methods, they must have different names. I have simply appended _err to all method names.

I invite bike shedding on the method names and the entire approach.

Defining an extension trait is easier than creating a wrapper struct (unless you're abusing Deref), but also more error prone for the user as they have to make sure to call the right method manually.

[andrewhickman]

The extension trait approach sounds fine to me, although its a bit unfortunate that we have to rename the functions

The _err suffix doesn't read very well to me - e.g. canonicalize_err sounds like its canonicalizing an error. Maybe fs_err_* would be clearer?

[Emerentius]

Yeah, I can rename it to fs_err_*".

Like with the structs in std::fs, creating a wrapper would give a nicer interface (same method names) but unlike the fs types, Path has a much larger API surface to replicate and I don't mean just the methods but also all the trait impls.

[andrewhickman]

Thanks!