asn1: Circular types and Topo. sort declarations

Many external ASN.1 modules that we have imported over time define types
like this:

  Foo ::= SEQUENCE { bar Bar }
  Bar ::= SEQUENCE { aMember INTEGER }

and before this change one had to re-order the definitions so that the
one for `Bar` came first.  No more.

We can now have out of order definitions in ASN.1 modules and the
compiler will topologically sort output C type declarations so that one
no longer has to manually sort types in ASN.1 modules when importing
them.

Besides that, it is now possible to create circular data types using
OPTIONAL since we generate such fields as pointers (which can then be
pointers to incomplete struct declarations):

  Circular ::= SEQUENCE {
          name UTF8String,
          next Circular OPTIONAL
  }

Circular types aren't necessarily useful, but they have been used in the
past.  E.g., the rpc.mountd protocol uses a circular type as a linked
list -- it should just have used an array, of course, as that's
semantically equivalent but more space efficient in its encoding, but
the point is that such types exist out there.

lib/asn1/gen_template.c

@@ -879,6 +879,12 @@ gen_template_import(const Symbol *s)
     gen_extern_stubs(f, s->gen_name);
 }
 
+void
+generate_template_type_forward(const char *name)
+{
+    fprintf(get_code_file(), "extern const struct asn1_template asn1_%s[];\n", name);
+}
+
 static void
 generate_template_type(const char *varname,
 		       const char **dupname,
@@ -900,8 +906,6 @@ generate_template_type(const char *varname,
     if (type->type == TTag)
 	implicit = (type->tag.tagenv == TE_IMPLICIT);
 
-    fprintf(get_code_file(), "extern const struct asn1_template asn1_%s[];\n", tl->name);
-
     template_members(&tl->template, basetype, name, type, optional, implicit, isstruct, need_offset);
 
     /* if its a sequence or set type, check if there is a ellipsis */