Accept AsRef<Path> instead of PathBuf to mirror std

[Erk-]

This is a breaking change, but I think it will make the API nicer and more efficient since you no longer need to pass a owned string to the new method.

For reference the std library does it in the same way: https://doc.rust-lang.org/stable/std/fs/struct.File.html#method.open

This change will mean that you can do the following:

PpsDevice::new("/dev/pps0")?;