Implement basic Chao bound

Author: CyberiaResurrection

src/hypofuzz/hy.py

@@ -399,6 +399,13 @@ def _json_description(self) -> Report:
                 ls for _, ls in self.pool.interesting_examples.values()
             ]
             del report["since new cov"]
+
+        if self.ninputs >= 10 and "" == report["note"]:
+            singletons = self.pool.singletons
+            doubletons = self.pool.doubletons
+            offset = singletons * (singletons-1) / 2 if 0 == doubletons else singletons * singletons / (2 * doubletons)
+            offset = int(offset) + 1
+            report["est. branches"] = report["branches"] + offset
         return report
 
     @property