Core: Add constexpr constructors/operators to math structs

• Begin integrating `constexpr` on math tests; use `static_assert` where appropriate

modules/mobile_vr/mobile_vr_interface.cpp

@@ -135,7 +135,6 @@ void MobileVRInterface::set_position_from_sensors() {
 	// few things we need
 	Input *input = Input::get_singleton();
 	Vector3 down(0.0, -1.0, 0.0); // Down is Y negative
-	Vector3 north(0.0, 0.0, 1.0); // North is Z positive
 
 	// make copies of our inputs
 	bool has_grav = false;