Remove nalgebra dependency

- Implement our own `Point` and `Vector` types
  - Make `Rectangle` not generic

src/vector.rs

@@ -1,2 +1,15 @@
 /// A 2D vector.
-pub type Vector = nalgebra::Vector2<f32>;
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub struct Vector {
+    pub x: f32,
+    pub y: f32,
+}
+
+impl Vector {
+    /// Creates a new [`Vector`] with the given components.
+    ///
+    /// [`Vector`]: struct.Vector.html
+    pub fn new(x: f32, y: f32) -> Self {
+        Self { x, y }
+    }
+}