Add a Counter.reset function (#2678)

contracts/mocks/CountersImpl.sol

@@ -20,4 +20,8 @@ contract CountersImpl {
     function decrement() public {
         _counter.decrement();
     }
+
+    function reset() public {
+        _counter.reset();
+    }
 }

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;
+    }
 }

contracts/utils/README.adoc

@@ -10,7 +10,7 @@ The {Address}, {Arrays} and {Strings} libraries provide more operations related
 
 For new data types:
 
- * {Counters}: a simple way to get a counter that can only be incremented or decremented. Very useful for ID generation, counting contract activity, among others.
+ * {Counters}: a simple way to get a counter that can only be incremented, decremented or reset. Very useful for ID generation, counting contract activity, among others.
  * {EnumerableMap}: like Solidity's https://solidity.readthedocs.io/en/latest/types.html#mapping-types[`mapping`] type, but with key-value _enumeration_: this will let you know how many entries a mapping has, and iterate over them (which is not possible with `mapping`).
  * {EnumerableSet}: like {EnumerableMap}, but for https://en.wikipedia.org/wiki/Set_(abstract_data_type)[sets]. Can be used to store privileged accounts, issued IDs, etc.