Move type operations to another structure

Every type has free, print, check_constraints, ber_decoder, der_encoder,
xer_decoder, xer_encoder, uper_decoder, uper_encoder and outmost_tag
operations. We move them out to a separate structure asn_TYPE_operation_t.

Combined with previous logic simplification, these operations are based
on ASN.1 basic types, constructed types and string types. So we can
reduce the space occupied by asn_TYPE_descriptor_t variables.
diff --git a/skeletons/xer_encoder.c b/skeletons/xer_encoder.c
index e52ee15..25b3bfa 100644
--- a/skeletons/xer_encoder.c
+++ b/skeletons/xer_encoder.c
@@ -25,7 +25,7 @@
 
 	ASN__CALLBACK3("<", 1, mname, mlen, ">", 1);
 
-	tmper = td->xer_encoder(td, sptr, 1, xer_flags, cb, app_key);
+	tmper = td->op->xer_encoder(td, sptr, 1, xer_flags, cb, app_key);
 	if(tmper.encoded == -1) return tmper;
 
 	ASN__CALLBACK3("</", 2, mname, mlen, ">\n", xcan);