crypto/algorithms/base.js: Convert to es6

Convert base to an es6 module with es6 classes, for clarity and to help with
jsdoccing.

Complications are:

* jsdoc gets confused by `export class`, so the exports are separated.

* turns out that extending Error is a bit difficult, so instanceof doesn't work
  on derived Error classes. This only really affects us in one place (app-side
  code shouldn't be doing instanceofs anyway), so just use `name` instead.

src/client.js

@@ -567,7 +567,7 @@ function _decryptEvent(client, event) {
         console.warn(
             `Error decrypting event (id=${event.getId()}): ${e}`,
         );
-        if (!(e instanceof Crypto.DecryptionError)) {
+        if (e.name !== "DecryptionError") {
             throw e;
         }
         _badEncryptedMessage(event, e.message);