Add FastRandomContext::rand256() and ::randbytes()

FastRandomContext now provides all functionality that the real Rand* functions
provide.

src/random.h

@@ -110,9 +110,15 @@ public:
         }
     }
 
+    /** Generate random bytes. */
+    std::vector<unsigned char> randbytes(size_t len);
+
     /** Generate a random 32-bit integer. */
     uint32_t rand32() { return randbits(32); }
 
+    /** generate a random uint256. */
+    uint256 rand256();
+
     /** Generate a random boolean. */
     bool randbool() { return randbits(1); }
 };