Add a few intrinsics contracts

[celinval]

Here are a few problems that I bumped into:

1. Verification currently does not pass for write_bytes, copy and copy_nonoverlapping.
   • Issue Support for slices in modifies clauses kani#2908.
2. The case where a pointer is dangling needed to be removed.
   • Kani cannot make assumptions on pointer validity (Add support for non-deterministic pointer kani#2300).
   • I don't know if exposing __CPROVER_is_fresh would be sound. From the documentation, it sounds like it will allocate fresh memory for each pointer. However, we would also like to cover the case where these pointers overlap.
3. Actual intrinsics are very hard to verify with Kani. The cases we can verify are those that have wrappers around the actual intrinsic.
   • Issue Add contract support to intrinsics kani#3345

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT licenses.

[rahulku]

is this waiting for something?

[celinval]

Yes, I need to update the branch and tests.