Core: Add constexpr constructors/operators to math structs

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

editor/import/3d/collada.cpp

@@ -208,7 +208,6 @@ Vector<float> Collada::AnimationTrack::get_value_at_time(float p_time) const {
 
 				Vector<float> ret;
 				ret.resize(16);
-				Transform3D tr;
 				// i wonder why collada matrices are transposed, given that's opposed to opengl..
 				ret.write[0] = interp.basis.rows[0][0];
 				ret.write[1] = interp.basis.rows[0][1];