Encoding:
    SILC_ASN1_BIT_STRING(str, str_len)
    SILC_ASN1_BIT_STRING_T(opts, tag, str, str_len)

    Decoding:
    SILC_ASN1_BIT_STRING(&str, &str_len)
    SILC_ASN1_BIT_STRING_T(opts, tag, &str, &str_len)

    Macro used to encode or decode bit string.  The string length in
    encoding must be in bits (bytes * 8).  The decoded length is in
    bits as well.  The string type is unsigned char and string length
    SilcUInt32.

    The `opts' is SilcAsn1Options.  The `tag' is a tag number.