Restore BN export (#428)

The `BN` export was removed in #398. I hadn't anticipated at the time
that this would constitute a breaking change. It turns out that this
export was relied upon in mobile in one place.

It has been temporarily restored, along with a comment that suggests
we remove it in the next major version.

src/assets/TokenBalancesController.test.ts

@@ -7,7 +7,7 @@ import { PreferencesController } from '../user/PreferencesController';
 import { AssetsController } from './AssetsController';
 import { Token } from './TokenRatesController';
 import { AssetsContractController } from './AssetsContractController';
-import TokenBalancesController from './TokenBalancesController';
+import { BN as exportedBn, TokenBalancesController } from './TokenBalancesController';
 
 const MAINNET_PROVIDER = new HttpProvider('https://mainnet.infura.io');
 
@@ -28,6 +28,10 @@ describe('TokenBalancesController', () => {
     sandbox.restore();
   });
 
+  it('should re-export BN', () => {
+    expect(exportedBn).toEqual(BN);
+  });
+
   it('should set default state', () => {
     expect(tokenBalances.state).toEqual({ contractBalances: {} });
   });

src/assets/TokenBalancesController.ts

@@ -5,6 +5,9 @@ import AssetsController from './AssetsController';
 import { Token } from './TokenRatesController';
 import { AssetsContractController } from './AssetsContractController';
 
+// TODO: Remove this export in the next major release
+export { BN };
+
 /**
  * @type TokenBalancesConfig
  *