Expose fewer public types

[qinsoon]

Currently mmtk-core exposes a lot of modules/types/methods/fields as public. Some are careless decisions. This brings the burden to us that some internal refactoring would end up changing the bindings as well. We should do a systematic pass and revisit each public item carefully, and only expose what is necessary.

• Ideally a binding should only need to use API functions we provide.
• We should only expose types that are used in those API functions.
• We should hide details in a module, and only expose certain types from a module.
• We should try not expose fields.

[qinsoon]

We should see if https://crates.io/crates/depub is helpful for our goal.