api: Use AsRef<Path> instead of &Path

[posborne]

This matches the convention used in std and makes the API simpler when calling open() as you can just provide a &str or String and it will be converted into a &Path autmmatically. This change is backwards compatible, so a full semver bump should not be required.

You can see this pattern in std in a number of places:

• https://doc.rust-lang.org/std/fs/struct.File.html#method.open
• https://doc.rust-lang.org/std/fs/fn.read_dir.html
• ... (basically everything that uses a path) ...

[vhbit]

Thanks, merged as ee2ff8a81ab379943000d77eca164cdd853a4545