Add support for float_to_int_unchecked

[celinval]

          Hi @carolynzech @celinval @feliperodri @zhassan-aws 

When I was testing my harness for f32::to_int_unchecked, I encountered the error indicating that float_to_int_unchecked is not currently supported by Kani, as shown below. Is it possible to support it?

SUMMARY:
 ** 1 of 1277 failed (1276 undetermined)
Failed Checks: float_to_int_unchecked is not currently supported by Kani. Please post your example at https://github.com/model-checking/kani/issues/new/choose
 File: "/Users/yew005/Docs/Academic/CMU/Fall24/practicum/verify-rust-std/library/core/src/convert/num.rs", line 30, in <f32 as convert::num::FloatToInt<i32>>::to_int_unchecked

VERIFICATION:- FAILED
** WARNING: A Rust construct that is not currently supported by Kani was found to be reachable. Check the results for more details.
Verification Time: 5.8839436s

Summary:
Verification failed for - num::verify::checked_to_int_unchecked_f32
Complete - 0 successfully verified harnesses, 1 failures, 1 total.

library/core/src/convert/num.rs Line 20 to 35:

20 macro_rules! impl_float_to_int {
21     ($Float:ty => $($Int:ty),+) => {
22         #[unstable(feature = "convert_float_to_int", issue = "67057")]
23         impl private::Sealed for $Float {}
24         $(
25             #[unstable(feature = "convert_float_to_int", issue = "67057")]
26             impl FloatToInt<$Int> for $Float {
27                 #[inline]
28                 unsafe fn to_int_unchecked(self) -> $Int {
29                     // SAFETY: the safety contract must be upheld by the caller.
30                     unsafe { crate::intrinsics::float_to_int_unchecked(self) }
31                 }
32            }
33         )+
34     }
35 }

Test harness:

#[kani::proof_for_contract(f32::to_int_unchecked)]
pub fn checked_to_int_unchecked_f32() {
    let num1: f32 = kani::any::<f32>();

    let result = unsafe { num1.to_int_unchecked::<i32>() };

    assert_eq!(result, num1 as i32);
}

Contracts added to f32::to_int_unchecked (in library/core/src/num/f32.rs):

/// # Safety
///
/// The value must:
///
/// * Not be `NaN`
/// * Not be infinite
/// * Be representable in the return type `Int`, after truncating off its fractional part
/// ...
#[requires(!self.is_nan() && !self.is_infinite())]
#[requires(self >= Self::MIN && self <= Self::MAX)]
pub unsafe fn to_int_unchecked<Int>(self) -> Int
where
    Self: FloatToInt<Int>,
{ ... }

Thank you very much!

Originally posted by @Yenyun035 in model-checking/verify-rust-std#59 (comment)