Changes to Counter (#1332)

* rename Index.currentId to current

* use += operator for clarity

* rename Counter.Index to Counter.Counter

* move Counter to drafts

(cherry picked from commit 3e55408cb5)

contracts/drafts/Counter.sol

@@ -4,10 +4,10 @@ pragma solidity ^0.4.24;
 /**
  * @title Counter
  * @author Matt Condon (@shrugs)
- * @dev Provides an incrementing uint256 id acquired by the `Index#next` getter.
+ * @dev Provides an incrementing uint256 id acquired by the `Counter#next` getter.
  * Use this for issuing ERC721 ids or keeping track of request ids, anything you want, really.
  *
- * Include with `using Counter for Counter.Index;`
+ * Include with `using Counter for Counter.Counter;`
  * @notice Does not allow an Id of 0, which is popularly used to signify a null state in solidity.
  * Does not protect from overflows, but if you have 2^256 ids, you have other problems.
  * (But actually, it's generally impossible to increment a counter this many times, energy wise
@@ -15,15 +15,15 @@ pragma solidity ^0.4.24;
  */
 library Counter {
 
-  struct Index {
+  struct Counter {
-    uint256 currentId; // default: 0
+    uint256 current; // default: 0
   }
 
-  function next(Index storage index)
+  function next(Counter storage index)
     internal
     returns (uint256)
   {
-    index.currentId = index.currentId + 1;
+    index.current += 1;
-    return index.currentId;
+    return index.current;
   }
 }

contracts/mocks/CounterImpl.sol

@@ -1,15 +1,15 @@
 pragma solidity ^0.4.24;
 
-import "../utils/Counter.sol";
+import "../drafts/Counter.sol";
 
 
 contract CounterImpl {
-  using Counter for Counter.Index;
+  using Counter for Counter.Counter;
 
   uint256 public theId;
 
   // use whatever key you want to track your counters
-  mapping(string => Counter.Index) private _counters;
+  mapping(string => Counter.Counter) private _counters;
 
   function doThing(string key)
     public