ModuleExport#identifier never used, so just use Element directly

[andy-hanson]

Noticed that this class was only used for a single member; so it makes more sense to just use that member directly and remove the class. (And it looks like that would have been the only place where Program exposed AST nodes.)

[dcodeIO]

I believe the reasoning here is that the identifier for an export can differ from the identifier of the element, for example in:

import { A as B } from "some/file";
export { B };

which makes the export use identifier B instead of the element's declared identifier A. To me it seems that .identifier should actually be read somewhere, which might be a bug. If not that, it might be there for diagnostics that just don't exist yet. Hmm...

[dcodeIO]

Superseded by #489, which updates the representation of module exports as well.