Introduce coercions for data instance decls

Mon Sep 18 19:07:30 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
  * Introduce coercions for data instance decls
  Tue Aug 22 20:33:46 EDT 2006  Manuel M T Chakravarty <chak@cse.unsw.edu.au>
    * Introduce coercions for data instance decls
    - data instance declarations implicitly generate a coercion moving between the
      representation type and family instance type.
    - The coercion is *implicitly* generated when type checking both source and 
      ifaces.  Ie, we don't safe it in ifaces - this is really exactly as newtype 
      coercions are handled.
    - The previous addition of the instance types to DataCons has been moved to 
      the representation TyCon.  This is more efficient as it is shared between all
      constructors of one representation tycon and it also gathers everything about
      data instances (family tycon, instance types, and coercion) in one place: the
      algTcParent field of TyCon.
    - The coercion is already used in the datacon wrappers, but not yet during type
      checking pattern matching of indexed data types.
    - The code has only been lightly tested, but doesn't seem to break features not
      related to indexed types.  For indexed data types only the pattern matching
      tc code (in TcPat.tcConPat) and some well-formedness checks are still 
      missing.  And there will surely be some bugs to fix.  (newtypes still require
      some more work.)
    
    	   ** WARNING: Interface file format changed! **
    	   **          Recompile from scratch!        **