Introduced XTypes and reworked the type graph infrastructure

[JohannesMeierSE]

This PR includes several issues we talked about during our last meetings.

• introduced PrimitiveType, ClassType, ... which extend Type
• introduced concrete types for edges and made TypeEdge abstract
• fixed bugs
• reduced the role of PrimitiveKind, ClassKind, ... accordingly
• TypirProblem is an interface to extend now instead of a type union in order to enable additional problems by users
• conversion: added structures to deal with transitivity and cycles, actual implementations of the algorithms are still missing (and will be added in another PR)
• first support for bottom types
• improved validation API
• introduced type.getName() to have a short string to show to users
• improved some details for FunctionTypes
• unified and extended logic to compare types with each other
• enforce uniqueness of new types, fixed calculation of uniqueness of functions
• refactorings, simplified code, comments

For the review, it might be easier to check the single commits.

[JohannesMeierSE]

Thank you @insafuhrmann and @Lotes a lot for your reviews!

I resolved most of your comments and pushed the changes.