Add more test cases for numeric host APIs (#66)

* Add test cases for bigInt host APIs

* Add test cases for bigDecimal fromString, plus and minus host APIs

* Add test cases for bigDecimal times and divideBy host APIs

* Throw an error on underflow or overflow in bigDecimal toString host API

* Avoid rounding of out of range bigDecimal values

* Use big number's byte size to set storage size of bigInt

* Add test cases for errors in bigDecimal host API

packages/graph-node/test/utils/index.ts

@@ -22,7 +22,8 @@ export const getDummyEventData = (): EventData => {
     uncleHash: ZERO_HASH,
     difficulty: '0',
     gasLimit: '0',
-    gasUsed: '0'
+    gasUsed: '0',
+    author: ZERO_ADDRESS
   };
 
   const tx = {