Add a hardware_destructive_interference_size const to the standard library

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http://www.eelis.net/c++draft/hardware.interference

constexpr size_t hardware_destructive_interference_size = implementation-defined;

This number is the minimum recommended offset between two concurrently-accessed objects to avoid additional performance degradation due to contention introduced by the implementation. It shall be at least alignof(max_align_t).

struct keep_apart {
  alignas(hardware_destructive_interference_size) atomic<int> cat;
  alignas(hardware_destructive_interference_size) atomic<int> dog;
};

constexpr size_t hardware_constructive_interference_size = implementation-defined;

This number is the maximum recommended size of contiguous memory occupied by two objects accessed with temporal locality by concurrent threads. It shall be at least alignof(max_align_t).

struct together {
  atomic<int> dog;
  int puppy;
};
struct kennel {
  // Other data members...
  alignas(sizeof(together)) together pack;
  // Other data members...
};
static_assert(sizeof(together) <= hardware_constructive_interference_size);

Start by reading the linked C++ draft section and the Rust standard-library contribution guidance to determine the appropriate API location and implementation-defined behavior. Define what tests are needed to verify both constants and their documented alignment or size guarantees; the work is done when the API and its semantics are accepted by the standard-library review process.