What gets done when printing an interface
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Basically, we do three things:

A) Create the import decls. For classes and values, this is easy. We
   filter the CE and GVE for all exported objects that were not declared 
   in the module. For types, this is a pain because we may have something
   which is exported and which refers to a type that isn't. For example,
   the interface
   interface C where
   ...
   f :: A -> B
   may export B, but A may be expected to come from somewhere else when
   C is imported. So, we have to go through all envs which have ranges that
   may refer to a type. This means the TCE, CE (the class op types), 
   GIE_inst (instance types) and GVE (types in the sigs). AND we have to
   filter out prelude defined types from the resulting list.

   Finally, we print the import decls, using the conventions that the renamer
   expects (no explicit constructors/ class ops, etc.)

B) Print the fixity decls for whatever constructors/ functions are exported

C) Print the rest of the decls needed. 
	1) Type decls - contents of TCE with export flags
	2) Class decls - contents of CE with export flags
	3) Instance decls - contents of GIE_inst that refer to either
                            an exported type or an exported class
                            (filter then print)
	4) Value decls - contents of GVE which are not constructors and
                         which have an export flag

Issues
~~~~~~

Type synonyms - to expand or not? Let's not, and complain if a type sig. is
	        used but not defined

Canonical form for interfaces - to get rid of perl post-processing!

Deriving for an abstract data type - shall we worry about this now or later?

Printing issues
~~~~~~~~~~~~~~~

It's convenient to make all ranges of environments know how to print themselves
(they do now) and decide whether to do so by looking at the export flag
in their Name fields. Presumably the constructors of a data type that is
exported abstractly will decide not to print themselves, so no special code
is needed.