[breaking] Move lerp() from InnerSpace to VectorSpace

Because it does not require dot product.
Along the way, fix the comment.

Fixes #471.

CHANGELOG.md

@@ -6,6 +6,9 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 
 ## [Unreleased]
 
+### Changed
+ - Move `lerp()` from `InnerSpace` to `VectorSpace`
+
 ## [v0.16.1] - 2018-03-21
 
 ### Added

src/structure.rs

@@ -181,6 +181,13 @@ where
 {
     /// The associated scalar.
     type Scalar: BaseNum;
+
+    /// Returns the result of linearly interpolating the vector
+    /// towards `other` by the specified amount.
+    #[inline]
+    fn lerp(self, other: Self, amount: Self::Scalar) -> Self {
+        self + ((other - self) * amount)
+    }
 }
 
 /// A type with a distance function between values.
@@ -261,13 +268,6 @@ where
         self * (magnitude / self.magnitude())
     }
 
-    /// Returns the result of linearly interpolating the magnitude of the vector
-    /// towards the magnitude of `other` by the specified amount.
-    #[inline]
-    fn lerp(self, other: Self, amount: Self::Scalar) -> Self {
-        self + ((other - self) * amount)
-    }
-
     /// Returns the
     /// [vector projection](https://en.wikipedia.org/wiki/Vector_projection)
     /// of the current inner space projected onto the supplied argument.