Expose ExportsWalker (#934)

Author: singpolyma-shopify

NOTICE

@@ -16,6 +16,7 @@ under the licensing terms detailed in LICENSE:
 * Willem Wyndham <willem@cs.umd.edu>
 * Bowen Wang <bowen@nearprotocol.com>
 * Emil Laine <laine.emil@gmail.com>
+* Stephen Paul Weber <stephen.weber@shopify.com>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:

src/definitions.ts

@@ -41,7 +41,7 @@ import {
 } from "./util";
 
 /** Walker base class. */
-abstract class ExportsWalker {
+export abstract class ExportsWalker {
 
   /** Program reference. */
   program: Program;