`&RelativePath` doesn't implement `Deserialize`

[Ontonator]

&RelativePath doesn't implement Deserialize but &std::path::Path does. I'm not too familiar with the inner workings of serde, but since both RelativePath and std::path::Path are both just wrappers around str and OsStr respectively, it seems like it should work. Currently, an extra allocation and copy must be performed when deserialising relative paths because a RelativePathBuf must be used instead.