Now respects BUILTIN types. Fixes #2.

* CHAR, FLOAT, INTEGER, IO and STRING are
  now exported as Char, Double, Integer, IO and String
  respectively.
* LIST and BOOL are ignored for the moment because exporting
  them is essentially the same as EXPORT_DATA
* NATURAL will not be exported because it wants Int or Integer
  both of which are signed. It is especially bad with converting
  from them: any negative input will be transformed to zero.

Note on performance: it is currently very slow because it reiterates
on all BUILTINs at every export instead of caching it.