Fix remaining warnings.

Most of these deserve to be revisited in the future as they
indicate that the C-FFI writer doesn't support everything.
Rename exports.dylan to library.dylan.
Remove outdated comment.
Re-organize: melange-c to melange-core.

Previously, there was a melange-c library in the melange-parser
directory. Both the library and the directory have now been
renamed to melange-core.
Call join() correctly for handling superclasses.
[doc] Fix some indentation / spacing.
[doc] Link to C-FFI docs.
[doc] Use haiku theme for now.