Use the power of the `AsRef` trait to take generic parameters for
several API functions. This makes the API more ergonomic because the
callers may pass in static `str` slices or references to owned `String`s
or even more exotic things like a `Cow`, all based on their particular
situation.
Update the tests and examples to use the most natural types they have
available.
Fixes#77. No existing code should break, as `&String` implements
`AsRef<str>` and `AsRef<Path>`
