Rewrite registry.h

This rewrites registry.h, removing all the macros and replacing it
with relatively ordinary template classes.  The result is less code
than the previous setup.  It replaces large macros with a relatively
straightforward C++ class, and now manages its own cleanup.

The existing type-safe "key" class is replaced with the equivalent
template class.  This approach ended up requiring relatively few
changes to the users of the registry code in gdb -- code using the key
system just required a small change to the key's declaration.

All existing users of the old C-like API are now converted to use the
type-safe API.  This mostly involved changing explicit deletion
functions to be an operator() in a deleter class.

The old "save/free" two-phase process is removed, and replaced with a
single "free" phase.  No existing code used both phases.

The old "free" callbacks took a parameter for the enclosing container
object.  However, this wasn't truly needed and is removed here as
well.

gdb/hppa-tdep.c

@@ -84,7 +84,8 @@ struct hppa_objfile_private
    that separately and make this static. The solib data is probably hpux-
    specific, so we can create a separate extern objfile_data that is registered
    by hppa-hpux-tdep.c and shared with pa64solib.c and somsolib.c.  */
-static const struct objfile_key<hppa_objfile_private> hppa_objfile_priv_data;
+static const registry<objfile>::key<hppa_objfile_private>
+  hppa_objfile_priv_data;
 
 /* Get at various relevant fields of an instruction word.  */
 #define MASK_5 0x1f