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

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

src/random.cpp

@@ -304,6 +304,26 @@ void FastRandomContext::RandomSeed()
     requires_seed = false;
 }
 
+uint256 FastRandomContext::rand256()
+{
+    if (bytebuf_size < 32) {
+        FillByteBuffer();
+    }
+    uint256 ret;
+    memcpy(ret.begin(), bytebuf + 64 - bytebuf_size, 32);
+    bytebuf_size -= 32;
+    return ret;
+}
+
+std::vector<unsigned char> FastRandomContext::randbytes(size_t len)
+{
+    std::vector<unsigned char> ret(len);
+    if (len > 0) {
+        rng.Output(&ret[0], len);
+    }
+    return ret;
+}
+
 FastRandomContext::FastRandomContext(const uint256& seed) : requires_seed(false), bytebuf_size(0), bitbuf_size(0)
 {
     rng.SetKey(seed.begin(), 32);