Convert src to ES6

The bulk of this is just export/import changes, though there's a couple pieces to highlight:
* We no longer use default exports. This is because it's discouraged by the JS community, though not in any official capacity.
* We now use `polyfillSuper` for some prototype inheritance because the tests, and sometimes webpack, break on "cannot call EncryptionAlgorithm without 'new'". It's very much a workaround, and definitely not needed when we use real classes.

There is some import shuffling to help keep the imports clean - this was done by my IDE.

src/crypto/verification/request/VerificationRequest.js

@@ -15,15 +15,10 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-import logger from '../../../logger';
-import RequestCallbackChannel from "./RequestCallbackChannel";
+import {logger} from '../../../logger';
+import {RequestCallbackChannel} from "./RequestCallbackChannel";
 import {EventEmitter} from 'events';
-import {
-    newUnknownMethodError,
-    newUnexpectedMessageError,
-    errorFromEvent,
-    errorFactory,
-} from "../Error";
+import {errorFactory, errorFromEvent, newUnexpectedMessageError, newUnknownMethodError} from "../Error";
 
 // the recommended amount of time before a verification request
 // should be (automatically) cancelled without user interaction
@@ -57,7 +52,7 @@ export const PHASE_DONE = 6;
  * send and receive verification events are put in `InRoomChannel` or `ToDeviceChannel`.
  * @event "change" whenever the state of the request object has changed.
  */
-export default class VerificationRequest extends EventEmitter {
+export class VerificationRequest extends EventEmitter {
     constructor(channel, verificationMethods, userId, client) {
         super();
         this.channel = channel;