library: replace super::super by crate import

Done with the following shell command:
`sed -e 's/super::super/crate/' -i $(git grep -l super::super)`.

Followed by a `cargo fmt --all`.

src/core/camera.rs

@@ -1,5 +1,5 @@
-use super::super::{Point, Vector};
 use super::film::Film;
+use crate::{Point, Vector};
 
 /// Represent an abstract camera to observe the scene.
 #[derive(Debug, PartialEq)]

src/core/film.rs

@@ -1,4 +1,4 @@
-use super::super::{Point, Vector};
+use crate::{Point, Vector};
 
 /// Represent an abstract camera film, to know where each pixel is in space.
 #[derive(Debug, PartialEq)]