Add support for multiple context types mixins

[chpill]

Implementation to illustrate my suggestion on https://github.com/tonsky/rum/issues/136

It works, using child-context-types and context-types to aggregate the different types correctly. But you have to be careful not to have a {:class-properties {:context-types ...}} in one of your mixins, as it will be applied last and override whatever you had in the top level :context-types (idem for :child-context-types)