[Bugfix Beta] Improve performance of Mixin creation by 50% -> 60% (when mixins are composed of multiple mixins)

* shape Mixin correctly (properties set in constructor)
* don’t pass arguments object around
* prefer new Mixin(args, properties) internally

This isn’t really going to have much of an impact by itself, but is part of a larger push  to improve the performance of our existing object model.
Merge pull request #9551 from stefanpenner/cleanup-mixin-constructor

[Bugfix Beta] Improve performance of Mixin creation by 50% -> 60% (when ...