Contracts & Harnesses for wrapping_shr

[MWDZ]

Towards #59

Changes
Added contracts for wrapping_shr (located in library/core/src/num/int_macros.rs and uint_macros.rs)
Added harnesses for wrapping_shr of each integer type
i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize --- 12 harnesses in total.
Revalidation
Per the discussion in #59, we have to build and run Kani from feature/verify-rust-std branch.
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. checked_wrapping_shl_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 harnesses should pass the default checks (1251 checks where 1 unreachable).

SUMMARY:
 ** 0 of 161 failed (1 unreachable)

VERIFICATION:- SUCCESSFUL
Verification Time: 0.32086188s

Complete - 12 successfully verified harnesses, 0 failures, 12 total.

Example of the unreachable check:

Check 9: num::<impl i8>::wrapping_shr.assertion.1
	 - Status: UNREACHABLE
	 - Description: "attempt to subtract with overflow"
	 - Location: library/core/src/num/int_macros.rs:2199:42 in function num::<impl i8>::wrapping_shr

[celinval]

Awesome! Thanks