Rewrite ASN.1 identifier/length parsing in CBS.

Rewrite ASN.1 identifier/length parsing in CBS.

Provide internal asn1_get_identifier_cbs() and asn1_get_length_cbs()
functions that are called from asn1_get_object_cbs(). Convert the existing
ASN1_get_object() function so that it calls asn1_get_object_cbs(), before
mapping the result into the API that it implements.

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