The duplicated methods per type `style style_XXX_class`, `generate_XXX` and `reference_XXX` were abstracted into a BaseTypeRegistry class.
Provided a specific XXXTypeRegistry for every required type and added it as an attribute to the ClassRegistry.

Made generate_enums a "private" method of EnumsPlugin.

Decoupled value parsing and code generation, by moving the generation of Ast-Elements into a separate class.

Changed the generation of deprecation-reasons to print the deprecation reason after `DEPRECATED` instead of the field description.

Separated ast-generation to static class.
Streamlined control flow inside generate_input_annotation.

A centralized ast-generator module should make the reuse of repeating structures easier and hide the actual code-generation from plugin-developers.

Optimized imports

Added tests for ObjectsPlugin that include primitive types and nested object types

The issue is described in the doctstring to ObjectTypeTestCaseGenerator.make_test_cases_union_field_value()

This partially duplicated old tests in test_mro/test_multiple_inheritance

Fixed bug in ObjectsPlugin._generate_type_body (introduced in last commit) that broke forward references by referencing the wrong "parent" in the subsequent call to generate_object_field_annotation.

Added union method to the AnnotationAstGenerator.
Refactored last method calls in ClassRegistry that slipped through during the first refactoring.