Add Exchange Proxy to Ganache snapshot (#2612)

* `@0x/contracts-zero-ex`: Expose migration tools.

* `@0x/contract-addresses`: Update ganache snapshot Exchange Proxy addresses

* `@0x/migrations`: Add Exchange Proxy migration

contracts/zero-ex/src/index.ts

@@ -41,3 +41,6 @@ export {
     TupleDataItem,
     StateMutability,
 } from 'ethereum-types';
+
+export * from './nonce_utils';
+export * from './migration';