Replace unsafe transmute to a cast

[vlad20012]

unsafe { mem::transmute::<i16, u16>(exp) } as u64 is equivalent to exp as u16 as u64, this can be verified with a simple program:

fn main() {
    for i in i16::MIN ..= i16::MAX {
        assert_eq!(
            unsafe { std::mem::transmute::<i16, u16>(i) } as u64,
            i as u16 as u64
        )
    }
}

[mbrubeck]

Thanks!