arith_uint256: modernize comparison operators

Since C++20, operator!= is implicitly defaulted using operator==, and
operator<, operator<=, operator>, and operator>= are defaulted using
operator<=>, so it suffices to just provide these two.

Author: sipa

src/arith_uint256.h

@@ -6,6 +6,7 @@
 #ifndef BITCOIN_ARITH_UINT256_H
 #define BITCOIN_ARITH_UINT256_H
 
+#include <compare>
 #include <cstdint>
 #include <cstring>
 #include <limits>
@@ -212,13 +213,8 @@ class base_uint
     friend inline base_uint operator<<(const base_uint& a, int shift) { return base_uint(a) <<= shift; }
     friend inline base_uint operator*(const base_uint& a, uint32_t b) { return base_uint(a) *= b; }
     friend inline bool operator==(const base_uint& a, const base_uint& b) { return memcmp(a.pn, b.pn, sizeof(a.pn)) == 0; }
-    friend inline bool operator!=(const base_uint& a, const base_uint& b) { return memcmp(a.pn, b.pn, sizeof(a.pn)) != 0; }
-    friend inline bool operator>(const base_uint& a, const base_uint& b) { return a.CompareTo(b) > 0; }
-    friend inline bool operator<(const base_uint& a, const base_uint& b) { return a.CompareTo(b) < 0; }
-    friend inline bool operator>=(const base_uint& a, const base_uint& b) { return a.CompareTo(b) >= 0; }
-    friend inline bool operator<=(const base_uint& a, const base_uint& b) { return a.CompareTo(b) <= 0; }
+    friend inline std::strong_ordering operator<=>(const base_uint& a, const base_uint& b) { return a.CompareTo(b) <=> 0; }
     friend inline bool operator==(const base_uint& a, uint64_t b) { return a.EqualTo(b); }
-    friend inline bool operator!=(const base_uint& a, uint64_t b) { return !a.EqualTo(b); }
 
     /** Hex encoding of the number (with the most significant digits first). */
     std::string GetHex() const;