This adds support for exporting class expressions, which was previously
TODO'd.
We now correctly set the binding name of exports to `"*default*"` if
they are unnamed. I'm not sure what the difference between the
`InitializationMode` kinds is, but using `Initialize` fixes a bunch of
tests.
Note that some export tests (e.g. `eval-export-dflt-expr-cls-named.js`)
still fail, as we don't set the "name" property of exported classes
correctly.
176 new passes on test262