Contracts and Harnesses for unchecked_neg

[Yenyun035]

Towards #59

Changes

• Added contracts for unchecked_neg (located in library/core/src/num/int_macros.rs)
• Added a harness for unchecked_neg of each signed integer type
  • i8, i16, i32, i64, i128, isize --- 6 harnesses in total.
• Fixed comments.

Revalidation

1. Per the discussion in Challenge 11: Safety of Methods for Numeric Primitive Types #59, we have to build and run Kani from feature/verify-rust-std branch.
2. To revalidate the verification results, run the following command. <harness_to_run> can be either num::verify to run all harnesses or num::verify::<harness_name> (e.g. check_unchecked_neg_i8) to run a specific harness.

kani verify-std  "path/to/library" \
    --harness <harness_to_run> \
    -Z unstable-options \
    -Z function-contracts \
    -Z mem-predicates

All default harnesses should pass.

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

[Yenyun035]

@zhassan-aws Thank you for your reviews! I have fixed the precondition for unchecked_neg and fixed some comments.

[Yenyun035]

@feliperodri @celinval @carolynzech May I request the second approval to merge this PR? Thank you very much :)

[celinval]

Thanks!

[Yenyun035]

@celinval @zhassan-aws Thank you! This PR can get merged now :)