refactor: executor methods into Executor class

This class is exported only to assist people in implementing their own executors without duplicating too much code and should be used only as last resort for cases such as experimental syntax or if certain features could not be contributed upstream.

It is still part of the internal API and is versioned, so any changes to it are never considered breaking changes. If you still need to support multiple versions of the library, please use the `versionInfo` variable for version detection.

Author: yaacovCR

src/execution/__tests__/subscribe-test.ts

@@ -14,7 +14,7 @@ import { GraphQLInt, GraphQLString, GraphQLBoolean } from '../../type/scalars';
 
 import { GraphQLAggregateError } from '../../error/GraphQLAggregateError';
 
-import { buildExecutionContext, createSourceEventStream } from '../execute';
+import { Executor } from '../execute';
 import { subscribe } from '../subscribe';
 
 import { SimplePubSub } from './simplePubSub';
@@ -410,8 +410,11 @@ describe('Subscription Initialization Phase', () => {
       const document = parse('subscription { foo }');
       const result = await subscribe({ schema, document });
 
-      const exeContext = buildExecutionContext({ schema, document });
-      expect(await createSourceEventStream(exeContext)).to.deep.equal(result);
+      const executor = new Executor();
+      const exeContext = executor.buildExecutionContext({ schema, document });
+      expect(await executor.createSourceEventStream(exeContext)).to.deep.equal(
+        result,
+      );
       return result;
     }