Check exports

Closing #562 again, and also checks that when a class is exported so are all its members, as suggested by @jdegoes.
Well, you could do

```
forM_ exps $ \e -> do
  checkTypesAreExported e
  checkClassMembersAreExported e
  checkClassesAreExported e
```

I don't think that's too bad. Lambdabot kindly offers the following:

```
forM_ exps $ flip mapM 
  [ checkTypesAreExported
  , checkClassMembersAreExported
  , checkClassesAreExported
  ] . flip id
```

which seems much worse.