Refactor all types in their own modules.

src/key.rs

@@ -0,0 +1,28 @@
+#[cfg(feature = "serialization")] use serde_derive::{Deserialize, Serialize};
+
+use crate::interpolation::Interpolation;
+
+/// A spline control point.
+///
+/// This type associates a value at a given interpolation parameter value. It also contains an
+/// interpolation hint used to determine how to interpolate values on the segment defined by this
+/// key and the next one – if existing.
+#[derive(Copy, Clone, Debug)]
+#[cfg_attr(feature = "serialization", derive(Deserialize, Serialize))]
+#[cfg_attr(feature = "serialization", serde(rename_all = "snake_case"))]
+pub struct Key<T, V> {
+  /// Interpolation parameter at which the [`Key`] should be reached.
+  pub t: T,
+  /// Held value.
+  pub value: V,
+  /// Interpolation mode.
+  pub interpolation: Interpolation<T>
+}
+
+impl<T, V> Key<T, V> {
+  /// Create a new key.
+  pub fn new(t: T, value: V, interpolation: Interpolation<T>) -> Self {
+    Key { t, value, interpolation }
+  }
+}
+