Rename Fixture to TestClassInstance.
Add TestClass, representing a test class under execution.  TypeBehaviors work on TestClass instead of a bare Type instance.
TypeBehaviorAction delegate accepts a TestClass instead of a bare Type instance, just like TypeBehavior does.
TestClass holds a reference to the Convention it takes part in, simplifying TypeBehavior and TypeBehaviorAction which previously accepted *both* a TestClass and a Convention as separate arguments.
TestClass holds a reference to the CaseExecutions representing the execution of the test class, simplifying TypeBehavior and TypeBehaviorAction which previously accepted *both* a TestClass and a CaseExecution[] as separate arguments.