Consider replacing PathBuf with Utf8PathBuf

[jplatte]

… in places such as https://github.com/danigm/epub-rs/blob/master/src/doc.rs#L74.

PathBuf isn't really very portable. serde happens to support it, but only by failing (de)serialization on non-utf8 data. camino offers UTF-8 validated path types. It seems like all the paths this crate deals with are read from XML that has to be valid UTF-8 (or UTF-16, which can be converted) anyways, so this should only improve usability and not break any weird niche use cases.