Implement a "lowest upper bound" algorithm

For type inference, it is important to determine the lowest upper bound of two types. The current implementation is called findCommonSuperClass which is fine but limited. For example, if two types do not have a common superclass but implement the same interface, findCommonSuperClass would return Object where it could return the interface.

The idea is to replace the current implementation with a smarter one which computes the lowest upper bound (LUB) of two classes.