Stop mixing signed and unsigned types in bit operations

Only use unsigned types in [gs]et_byte_in_int (atomic.hpp). In the code below, bitsIdx ought to be uint32_t instead of int.

```
inline uint32_t Atomic::CmpxchgByteUsingInt::set_byte_in_int(uint32_t n,
                                                             uint8_t b,
                                                             uint32_t idx) {
  int bitsIdx = BitsPerByte * idx;
  return (n & ~(static_cast<uint32_t>(0xff) << bitsIdx))
          | (static_cast<uint32_t>(b) << bitsIdx);
}

inline uint8_t Atomic::CmpxchgByteUsingInt::get_byte_in_int(uint32_t n,
                                                            uint32_t idx) {
  int bitsIdx = BitsPerByte * idx;
  return (uint8_t)(n >> bitsIdx);
}
```