Minor header cleanup.

libs/kimath/include/math/vector2d.h

@@ -29,8 +29,8 @@
 #define VECTOR2D_H_
 
 #include <limits>
-#include <iostream>
-#include <sstream>
+#include <ostream>
+#include <string>
 #include <type_traits>
 
 #include <math/util.h>
@@ -407,11 +407,7 @@ VECTOR2<T> VECTOR2<T>::Resize( T aNewLength ) const
 template <class T>
 const std::string VECTOR2<T>::Format() const
 {
-    std::stringstream ss;
-
-    ss << "( xy " << x << " " << y << " )";
-
-    return ss.str();
+    return std::string( "( xy " ) + std::to_string( x ) + " " + std::to_string( y ) + " )";
 }