Move const time memory comparison utils to ct_utils.h

[randombit]

No description provided.

[coveralls]

coverage: 91.707% (+0.003%) from 91.704% when pulling e872668 on jack/ct-refactor into 5073121 on master.

[reneme]

This could be based on ranges, no? Something along these lines, with CT::Mask<> fairy-dust sprinkled in:

Suggested change

	/**
	* Compare two arrays of equal size and return a Mask indicating if
	* they are equal or not. The mask is set if they are identical.
	*/
	template <typename T>
	inline CT::Mask<T> is_equal(const T x[], const T y[], size_t len) {
	volatile T difference = 0;
	for(size_t i = 0; i != len; ++i) {
	difference = difference | (x[i] ^ y[i]);
	}
	return CT::Mask<T>::is_zero(difference);
	}
	template <std::ranges::contiguous_range X, std::ranges::contiguous_range Y>
	requires std::same_as<std::ranges::range_value_t<X>, std::ranges::range_value_t<Y>>
	constexpr auto is_equal(X&& x, Y&& y)
	{
	volatile auto difference = static_cast<std::ranges::range_value_t<X>>(std::ranges::size(x) != std::ranges::size(y));
	auto ix = std::ranges::begin(x);
	auto iy = std::ranges::begin(y);
	// TODO C++23: std::views::zip(x, y)
	// future-me, please make sure that zip() is constant-time
	while(ix != std::ranges::end(x) && iy != std::ranges::end(y))
	{
	difference |= *ix ^ *iy;
	++ix;
	++iy;
	}
	return difference;
	}```

[randombit]

Maybe so. But tbh for this kind of code I like writing things in the most brain-dead way possible because it's usually (fairly) obvious what the compiler is going to do with it.

Some span based overloads here might be nice though. As with #3757 the goal here is primarily to remove usage of functions in mem_ops.h especially when they were already redundant with other functionality.