Use Class::Load(::XS)

Our Module::Loader does things Class::Load doesn't.

But:
  * Class::Load uses Module::Runtime, that we used before, therefore, it should be slower than before (one more indirection layer)
  * Class::Load can use Class::Load::XS if it is installed, but not sure how faster it will run

So, I am not sure of the relevance of this change.