Per X.690 (2002), Section 11.5, for DER, set and sequence values should not
include elements equal to their default value.

This was already handled by the UPER encoder, but not by the DER
encoder.

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.

"specifics" structures and the tables they point to.