Implement re-exports

[jmackie]

The syntax for this would be:

module Reexport.Example exports (
    -- Qualified names here denote re-exports
    SomeModule.foo,
    SomeModule.Bar,
    SomeModule.Baz(..),
);

import (some-package) SomeModule;

This will be useful for exposing package internals (related #28) along wit their documentation etc.