Mocking constants and associated types in MockVM

[qinsoon]

#1049 introduced MockVM, which can mock methods for VMBinding. MockVM stores closures (MockMethod) for each method, and dispatches the calls for each method to the mock closures. However, it does not allow mocking constants and associated types.

To support mocking constants and associated types, we will have to use macros to generate a new MockVM type each time. This is important to us if we want to have good code coverage from unit tests.