Add a Counter.reset function (#2678)

contracts/utils/Counters.sol

@@ -5,7 +5,7 @@ pragma solidity ^0.8.0;
 /**
  * @title Counters
  * @author Matt Condon (@shrugs)
- * @dev Provides counters that can only be incremented or decremented by one. This can be used e.g. to track the number
+ * @dev Provides counters that can only be incremented, decremented or reset. This can be used e.g. to track the number
  * of elements in a mapping, issuing ERC721 ids, or counting request ids.
  *
  * Include with `using Counters for Counters.Counter;`
@@ -35,4 +35,8 @@ library Counters {
             counter._value = value - 1;
         }
     }
+
+    function reset(Counter storage counter) internal {
+        counter._value = 0;
+    }
 }