Use spaceship op to cover relational ops for comparing two bi_t objects

include/bi.hpp

@@ -8,6 +8,7 @@ SPDX-License-Identifier: Apache-2.0
 
 #include <algorithm>
 #include <cassert>
+#include <compare>
 #include <iostream>
 #include <limits>
 #include <span>
@@ -126,15 +127,9 @@ class BI_API bi_t {
   bi_t& operator<<=(bi_bitcount_t shift);
   bi_t& operator>>=(bi_bitcount_t shift);
 
-  // Relational operators
-  bool operator<(const bi_t&) const noexcept;
-  bool operator>(const bi_t&) const noexcept;
-  bool operator<=(const bi_t&) const noexcept;
-  bool operator>=(const bi_t&) const noexcept;
-
-  // Equality operators
+  // Comparisons
+  std::strong_ordering operator<=>(const bi_t& other) const noexcept;
   bool operator==(const bi_t&) const noexcept;
-  bool operator!=(const bi_t&) const noexcept;
 
   // Bitwise operators
   bi_t operator~() const;

src/bi.cpp

@@ -6,6 +6,7 @@ SPDX-License-Identifier: Apache-2.0
 #include "bi.hpp"
 
 #include <array>
+#include <compare>
 #include <iostream>
 #include <version>
 #if defined(__cpp_lib_format)
@@ -432,39 +433,18 @@ int bi_t::cmp(const bi_t& x, const bi_t& y) noexcept {
 }
 
 /**
- *  @name Relational operators
+ *  @name Comparisons
  */
 ///@{
 
-bool bi_t::operator<(const bi_t& other) const noexcept {
-  return cmp(*this, other) < 0;
+std::strong_ordering bi_t::operator<=>(const bi_t& other) const noexcept {
+  return cmp(*this, other) <=> 0;
 }
 
-bool bi_t::operator>(const bi_t& other) const noexcept { return other < *this; }
-
-bool bi_t::operator<=(const bi_t& other) const noexcept {
-  return !(*this > other);
-}
-
-bool bi_t::operator>=(const bi_t& other) const noexcept {
-  return !(*this < other);
-}
-
-///@}
-
-/**
- *  @name Equality operators
- */
-///@{
-
 bool bi_t::operator==(const bi_t& other) const noexcept {
   return cmp(*this, other) == 0;
 }
 
-bool bi_t::operator!=(const bi_t& other) const noexcept {
-  return !(*this == other);
-}
-
 ///@}
 
 /**