This makes Vec3<T> perform more like the usual arithmetic types, in
order to avoid having to explicitly perform element wise conversions
when two Vec3<>s have different numeric types.

For instance:
  _i = int16_t() + int()                // typeof(_i) == int
  vi = Vec3<int16_t>() + Vec3<int>()    // typeof(vi) == Vec3<int>

  _d = 1 + 2.0                          // typeof(_d) == double
  vd = Vec3<int>(1) + Vec3<double>(2.0) // typeof(vd) == Vec3<double>

  vi = Vec3<int>(Vec3<double>())        // typeof(vi) == Vec3<int>
  vd = Vec3<double>(Vec3<int>())        // typeof(vd) == Vec3<double>

Be aware, however, that the usual implicit conversions can result
in a loss of precision:

        int _i = 1 + 1.5                          // _i == 2;
  Vec3<int> vi = Vec3<int>(1) + Vec3<double>(1.5) // vi == Vec3<int>(2)