Implemented SchemaMergingVisitor that implements the SchemaDifferenceVisitor interface to call the actual merging methods depending on their change type.
Replaced the ItemMerger export with individual methods that get invoked in the SchemaMergingVisitor.
SchemaItemMerger has been deleted. The merging order moved from SchemaItemMerger to the SchemaMergingWalker, all shared methods like updateSchemaItemKey moved in Utils.
Two skipped tests has been re-enabled again.
Other changes:
ClassItemDifference union has been renamed to AnyClassItemDifference
Refactoring of the Merger:
Other changes: