For some reason, in 6.5 the manual said you could put a class decl in
an interface file, but not an instance decl; whereas the implementation
was exactly the othe way round.

This patch makes it possible to put *both* class and instance decls
in an interface file. 

I also did a bit of re-factoring; comparing the declarations in the
hs-boot and hs file is now done by converting to IfaceSyn, because we
have good comparison operations for IfaceSyn already implemented.
This fixed a bug that previously let through an inconsistent declaration 
of a data type.

The remaining infelicity concerns "abstract" TyCons.  They are a bit
of a hack anyway; and Classes are not handled in the same way.  Need
to think about this, but I think it's probably ok as it stands.