Free mixin features

[iboB]

Is it a good idea to allow free mixin features. This would be an additional list of features when creating a type, which is not carried in by one of the participating mixins.

• On one hand, zero-sized mixins can do the same job
• However with free features the object buffer becomes smaller and we don't have to manage it

I'm so far leaning against this, but leaving it here for posterity