Add GenIdentity, an identity function for MappedShuffle.

We use this function in z_sendmany as part of the fix for #1779.

src/random.h

@@ -25,6 +25,11 @@ uint64_t GetRand(uint64_t nMax);
 int GetRandInt(int nMax);
 uint256 GetRandHash();
 
+/**
+ * Identity function for MappedShuffle, so that elements retain their original order.
+ */
+ int GenIdentity(int n);
+
 /**
  * Rearranges the elements in the range [first,first+len) randomly, assuming
  * that gen is a uniform random number generator. Follows the same algorithm as