Add examples for GenericPath methods.

I wanted to use `assert_eq` throughout, but that does not seem possible (see #15625)
Is there an easy way to use `path::SEP` that I'm missing? What I've come up with seems horrible to me:

    let v = Vec::from_slice(b"foo").append_one(SEP_BYTE).append(b"bar")
    let path = Path::new(v);