Implement graph-ts numbers, typeConversion and log host APIs (#64)

* Implement host api for bigint operations

* Implement type conversion APIs

* Avoid use of exports as variable name for conflict

* Implement log API with debug and levels

* Fix job-runnner hook error after stopping and starting chain

packages/graph-node/assembly/index.ts

@@ -72,3 +72,11 @@ export class Bar {
     return this.prop;
   }
 }
+
+export function testLog (): void {
+  log.debug('Debug message: {}, {}', ['value1', 'value2']);
+  log.info('Info message: {}', ['value1', 'value2']);
+  log.warning('Warning message', []);
+  log.error('Error message', []);
+  log.critical('Critical message', []);
+}