[rfc] trad-frame change

[Andrew Cagney <cagney@gnu.org>]

Think of this as a pseudo patch.

I'm looking to extend the trad-frame code so that it includes the interface:

   trad_frame_append (gdbarch, frame_type, frame_sniffer, frame_cache);

This would do all the housekeeping necessary to create a FRAME_TYPE 
unwinder implemented with FRAME_SNIFFER and FRAME_CACHE functions.  The 
second function would have the interface:

   struct trad_frame_cache
   {
     struct frame_id this_id;
     CORE_ADDR this_base;
     struct trad_frame *saved_regs;
   };

   void frame_cache (struct frame_info *next_frame, struct 
trad_frame_cache *this_cache);

and would be called _once_ to populate the entire trad-frame cache. 
After that frame ID and register requests would be handled directly.

My rationale is simple.  Having just churned out unwinders for two 
architectures I'm seeing a pattern that suggests this would make life 
easier:

- people in a hurry can use interface

- people wanting to tune their unwinder can use the lower-level interface

thoughts?
Andrew

PS: This has been suggested before, but not as a new layer above the 
existing unwinder code