Allow address to stay the same during shrinking

The current shrinking implementation has a bug/limitation: it always
tries to shrink the sender. In other words, if a transaction has a
sender that is not minimal, Echidna will try to reduce it. However,
the transaction might require a different sender to still cause
the assertion failure, and the fact that Echidna will unconditionally
try to lower it might cause the shrinking to get 'stuck'.

This allows the sender shrinker to keep the transaction source the
same with some probability, allowing the shrinking to proceed in
such cases.

Author: elopez

lib/Echidna/Shrink.hs

@@ -128,6 +128,6 @@ shrinkSender x = do
   let orderedSenders = List.sort $ Set.toList senderSet
   case List.elemIndex x.src orderedSenders of
     Just i | i > 0 -> do
-      sender <- uniform (take i orderedSenders)
+      sender <- uniform (take (i+1) orderedSenders)
       pure x{src = sender}
     _ -> pure x