Demystify a few magic numbers.

src/bignum.h

@@ -228,10 +228,13 @@ public:
     {
         std::vector<unsigned char> vch2(vch.size() + 4);
         unsigned int nSize = vch.size();
+        // BIGNUM's byte stream format expects 4 bytes of
+        // big endian size data info at the front
         vch2[0] = (nSize >> 24) & 0xff;
         vch2[1] = (nSize >> 16) & 0xff;
         vch2[2] = (nSize >> 8) & 0xff;
         vch2[3] = (nSize >> 0) & 0xff;
+        // swap data to big endian
         reverse_copy(vch.begin(), vch.end(), vch2.begin() + 4);
         BN_mpi2bn(&vch2[0], vch2.size(), this);
     }