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/QRCode.js

@@ -19,13 +19,8 @@ limitations under the License.
  * @module crypto/verification/QRCode
  */
 
-import Base from "./Base";
-import {
-    errorFactory,
-    newUserCancelledError,
-    newKeyMismatchError,
-    newUserMismatchError,
-} from './Error';
+import {VerificationBase as Base} from "./Base";
+import {errorFactory, newKeyMismatchError, newUserCancelledError, newUserMismatchError} from './Error';
 
 const MATRIXTO_REGEXP = /^(?:https?:\/\/)?(?:www\.)?matrix\.to\/#\/([#@!+][^?]+)\?(.+)$/;
 const KEY_REGEXP = /^key_([^:]+:.+)$/;