Implement bigDecimal host API (#62)

* Handle negative bigInt in store host API

* Increase size of bigInt in fromString API to support UInt256

* Implement bigDecimal divideBy host API

* Implement bigDecimal plus, minus and times host API

* Implement bigInt dividedByDecimal and mod host API

* Change BN js version to that used by ethers js

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

@@ -17,7 +17,11 @@ export const getDummyEventData = (): EventData => {
     stateRoot: ZERO_HASH,
     td: ZERO_HASH,
     txRoot: ZERO_HASH,
-    receiptRoot: ZERO_HASH
+    receiptRoot: ZERO_HASH,
+    uncleHash: ZERO_HASH,
+    difficulty: '0',
+    gasLimit: '0',
+    gasUsed: '0'
   };
 
   const tx = {
@@ -30,7 +34,8 @@ export const getDummyEventData = (): EventData => {
   return {
     block,
     tx,
-    eventParams: [],
+    inputs: [],
+    event: {},
     eventIndex: 0
   };
 };