Fix a few more issues with the std library

[celinval]

• Enable Kani to use fallback fn body for intrinsics, so they can be verified.
  • Until https://github.com/rust-lang/project-stable-mir/issues/79 is implemented, we have to check has_body and must_be_overridden
• Export all kani_macro definitions. Rename unstable to avoid conflict with the Rust standard library one.
• Dump stable mir body since transformations are made at that level.
  • I just did this as I was debugging things.

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