Contracts & Harnesses for to_int_unchecked

[Yenyun035]

Towards #59
❗ Depends on this WIP Kani Issue and this PR, as discussed in this thread in #59

Changes

• Added contracts for f{16,32,64,128}::to_int_unchecked (located in library/core/src/num/f32.rs)
• Added a macro for generating to_int_unchecked harnesses
• Added harnesses for to_int_unchecked of each integer type
  • i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize --- 12 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. checked_f32_to_int_unchecked_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

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