add readme for captcha, and enhanced performance

utils/captcha/siprng_test.go

@@ -0,0 +1,19 @@
+package captcha
+
+import "testing"
+
+func TestSiphash(t *testing.T) {
+	good := uint64(0xe849e8bb6ffe2567)
+	cur := siphash(0, 0, 0)
+	if cur != good {
+		t.Fatalf("siphash: expected %x, got %x", good, cur)
+	}
+}
+
+func BenchmarkSiprng(b *testing.B) {
+	b.SetBytes(8)
+	p := &siprng{}
+	for i := 0; i < b.N; i++ {
+		p.Uint64()
+	}
+}