Introduce MockVM

[qinsoon]

This PR introduces MockVM, a type that implements VMBinding and allows users to control the behavior of each method for testing. This PR also moves all the tests in the current DummyVM to MockVM, and removes the current DummyVM.

This PR closes #99. Note that the current MockVM implementation does not allow changing constants or associated types in VMBinding -- I would suggest we create another issue to track this problem.

Changes:

• Introduce MockVM, and write all the current DummyVM tests with MockVM.
• Remove DummyVM, and remove ./examples which uses DummyVM.
• Change CI scripts to run tests with MockVM.
• Remove pub visibility for some modules. Those modules were exposed so we can test them from DummyVM. As now MockVM is a part of the main crate, we can test private items. We no longer need pub for those modules.

[wks]

I think the MockAny may not work because the original closures are supposed to accept objects that are implemented with different type arguments. But this fact still doesn't prevent us from doing mock tests for those methods. I have given concrete examples in in-line comments. For the root-scanning methods, we simply let the closure provide the list of roots instead of making a dyn callback objects from it to call. Similarly, for the weak reference processing methods, we just let the closure list the object references to be traced.

However, I tried to write a mock test case for testing the ScanVMSpecificRoots work packet, but failed because I found it hard to instantiate a concrete ProcessEdgesWorkRootWorkFactory without knowing the plan.

[wks]

LGTM