Merge bitcoin/bitcoin#33332: common: Make arith_uint256 trivially copyable

653a984 common: Make arith_uint256 trivially copyable (Fabian Jahr)

Pull request description:

  Makes `arith_uint256`/`base_uint` trivially copyable by removing the custom copy constructor and copy assignment operators. Removing of the custom code should not result in a change of behavior since `base_uint` contains a simple array of `uint32_t` and compiler generated versions of the code could be better optimized.

  This was suggested by maflcko here: bitcoin/bitcoin#30469 (review)

ACKs for top commit:
  Raimo33:
    ACK 653a984
  l0rinc:
    ACK 653a984
  achow101:
    ACK 653a984
  hodlinator:
    re-ACK 653a984

Tree-SHA512: 38db5220a2cf773c0c5fb5591671e329b6b87458d972db4f5f3f98c025ec329a8c39b32b5bc24ef8b50b1002b43bb248d8b35aa1c9a56c68c6bbd1d470485bd7

Author: achow101

src/arith_uint256.h

@@ -37,20 +37,8 @@ class base_uint
             pn[i] = 0;
     }
 
-    base_uint(const base_uint& b)
-    {
-        for (int i = 0; i < WIDTH; i++)
-            pn[i] = b.pn[i];
-    }
-
-    base_uint& operator=(const base_uint& b)
-    {
-        if (this != &b) {
-            for (int i = 0; i < WIDTH; i++)
-                pn[i] = b.pn[i];
-        }
-        return *this;
-    }
+    base_uint(const base_uint& b) = default;
+    base_uint& operator=(const base_uint& b) = default;
 
     base_uint(uint64_t b)
     {
@@ -272,6 +260,9 @@ class arith_uint256 : public base_uint<256> {
     friend arith_uint256 UintToArith256(const uint256 &);
 };
 
+// Keeping the trivially copyable property is beneficial for performance
+static_assert(std::is_trivially_copyable_v<arith_uint256>);
+
 uint256 ArithToUint256(const arith_uint256 &);
 arith_uint256 UintToArith256(const uint256 &);