Add to_path_buf() method for RelPath

[regexident]

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:

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:

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)

[Veykril]

@bors r+

[bors]

📌 Commit 0005794 has been approved by Veykril

It is now in the queue for this repository.

[bors]

⌛ Testing commit 0005794 with merge 0ac05c0...

[bors]

☀️ Test successful - checks-actions
Approved by: Veykril
Pushing 0ac05c0 to master...