crypto: replace CountBits with std::bit_width

bit_width is a drop-in replacement with an exact meaning in c++, so there is
no need to continue testing/fuzzing/benchmarking.

src/random.h

@@ -11,6 +11,7 @@
 #include <span.h>
 #include <uint256.h>
 
+#include <bit>
 #include <cassert>
 #include <chrono>
 #include <cstdint>
@@ -203,7 +204,7 @@ public:
     {
         assert(range);
         --range;
-        int bits = CountBits(range);
+        int bits = std::bit_width(range);
         while (true) {
             uint64_t ret = randbits(bits);
             if (ret <= range) return ret;