allOf types do not have all the correct instanceofs

[sakari]

makeAllOf uses the last type in allOf as the class to instantiate. This is a bit incorrect as then you cannot do X instanceof T for allOf: T, K. Also the types of the fields suffer from this issue.

Instead we should I guess merge the type definitions recursively and set the instanceof manually to the new object with Symbol.hasInstance

see smartlyio/oats-runtime#57 for related discussion on dangers of classes