Add eq and ne operators for compare utility.

[Benzinnos]

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[jwellbelove]

Maybe add the '<=>' operator for C++20 too?

[Benzinnos]

Maybe add the '<=>' operator for C++20 too?

1. So, are we really need spaceship operator? What the use case? We have already all 6 comparing operators.
2. If we are need this, possible implementations:

typedef int cmp_result_type;

static cmp_result_type cmp(first_argument_type lhs, second_argument_type rhs)
{
  if (lt(lhs, rhs)) {
    return -1;
  }

 if (gt(lhs, rhs)) {
    return 1;
  }

  return 0;
}

Or, if we can use std::*_ordering types:

typedef std::strong_ordering cmp_result_type;

static cmp_result_type cmp(first_argument_type lhs, second_argument_type rhs)
{
  if (lt(lhs, rhs)) {
    return std::strong_ordering::less;
  }

  if (gt(lhs, rhs)) {
    return std::strong_ordering::greater;
  }

  return std::strong_ordering::equivalent;
}

[jwellbelove]

So, are we really need spaceship operator? What the use case? We have already all 6 comparing operators.
Well, the struct is called 'compare' and I thought that the three way compare would be the only one missing from the set after this change.