Merge #12564: [arith_uint256] Avoid unnecessary this-copy using prefix operator

22b4aae [arith_uint256] Avoid unnecessary this-copy using prefix operator (Karl-Johan Alm)

Pull request description:

  I noticed while profiling a related project that `operator-()` actually calls the `base_uint` constructor, which is because the postfix operator version of `operator++` (used in `operator-()`) creates a copy of `this` and returns it.

Tree-SHA512: d9a2665caa3d93f064cdeaf1c6fada101b9943bb53d93ccac6d9a0edac20279d2e921349e30239039c71e0a9629e45c29ec9f10d8d7499e936cdba6cb7c3c3eb

Author: laanwj

src/arith_uint256.h

@@ -85,7 +85,7 @@ class base_uint
         base_uint ret;
         for (int i = 0; i < WIDTH; i++)
             ret.pn[i] = ~pn[i];
-        ret++;
+        ++ret;
         return ret;
     }