Fix CriAttributes Parser

[bkstein]

The parser implementation didn't work for CSR's containing a second attribute. The parser function was re-implemented and should work now according to RFC 2986, section 4.1, which defines multiple attributes:

CertificationRequestInfo ::= SEQUENCE {
        version       INTEGER { v1(0) } (v1,...),
        subject       Name,
        subjectPKInfo SubjectPublicKeyInfo{{ PKInfoAlgorithms }},
        attributes    [0] Attributes{{ CRIAttributes }}
   }

   SubjectPublicKeyInfo { ALGORITHM : IOSet} ::= SEQUENCE {
        algorithm        AlgorithmIdentifier {{IOSet}},
        subjectPublicKey BIT STRING
   }

   PKInfoAlgorithms ALGORITHM ::= {
        ...  -- add any locally defined algorithms here -- }

   Attributes { ATTRIBUTE:IOSet } ::= SET OF Attribute{{ IOSet }}

   CRIAttributes  ATTRIBUTE  ::= {
        ... -- add any locally defined attributes here -- }

   Attribute { ATTRIBUTE:IOSet } ::= SEQUENCE {
        type   ATTRIBUTE.&id({IOSet}),
        values SET SIZE(1..MAX) OF ATTRIBUTE.&Type({IOSet}{@type})
   }

Special thanks to my rubber duck debugger @fzpp, who gave crucial hints.

[chifflier]

PR looks good to me, thanks!