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/RequestCallbackChannel.js

@@ -19,7 +19,7 @@ limitations under the License.
  * This way, the VerificationRequest can update its state based on events sent by the verifier.
  * Anything that is not sending is just routing through to the wrapped channel.
  */
-export default class RequestCallbackChannel {
+export class RequestCallbackChannel {
     constructor(request, channel) {
         this._request = request;
         this._channel = channel;