Add `to_path_buf()` method for `RelPath` There seems to be no ergonomic way to obtain a `RelPathBuf` from a corresponding `&RelPath` at the moment, making the latter sort of a dead end. The `AbsPath` type provides the following: ```rust impl AbsPath { // ... /// Equivalent of [`Path::to_path_buf`] for `AbsPath`. pub fn to_path_buf(&self) -> AbsPathBuf { AbsPathBuf::try_from(self.0.to_path_buf()).unwrap() } // ... } ``` So I took the liberty of adding a corresponding equivalent for `RelPath: ```rust impl RelPath { // ... /// Equivalent of [`Path::to_path_buf`] for `RelPath`. pub fn to_path_buf(&self) -> RelPathBuf { RelPathBuf::try_from(self.0.to_path_buf()).unwrap() } // ... } ``` (the change is motivated by an outside use of the `ra_ap_paths` crate that would benefit from being able to use `RelPath` and `AbsPath` over `Path`)