[backend] add more ASN1 helpers for packing and unpacking

- asn1_enumerated, asn1_string, asn1_bytes, asn1_bits_list
- asn1_unpack_integer, asn1_unpack_set, asn1_unpack_bytes, asn1_unpack_octet_string,
  asn1_unpack_string, asn1_unpack_bits_list, asn1_unpack_boolean, asn1_unpack_tagged