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

@@ -0,0 +1,29 @@
+pragma solidity ^0.4.24;
+
+
+/**
+ * @title Counter
+ * @author Matt Condon (@shrugs)
+ * @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.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
+ * so it's not something you have to worry about.)
+ */
+library Counter {
+
+  struct Counter {
+    uint256 current; // default: 0
+  }
+
+  function next(Counter storage index)
+    internal
+    returns (uint256)
+  {
+    index.current += 1;
+    return index.current;
+  }
+}