AWS-Team 2: Contracts & Harnesses for unchecked_add

[Yenyun035]

Towards #59

Changes

• Added contracts for unchecked_add (located in library/core/src/num/int_macros.rs and uint_macros.rs)
• Added a harness for unchecked_add of each integer type
  • i8, i16, i32, i64, i128, u8, u16, u32, u64, u128 --- 10 harnesses in total.

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_add_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 10 harnesses should pass the same 1229 checks:

SUMMARY:
 ** 0 of 1229 failed

VERIFICATION:- SUCCESSFUL
Verification Time: 2.4755886s

Complete - 10 successfully verified harnesses, 0 failures, 10 total.

Sidenotes

I ran rustfmt on mod.rs, int_macros.rs, and uint_macros.rs, so there might be format changes to existing code.

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