See `asn1: Add --decorate=... for internal bookkeeping`, which adds an
option to `asn1_compile` for decorating SET/SEQUENCE types with fields
that are neither encoded nor decoded, but which _are_ copied and freed.
We'll use this to add name attributes to the `Principal` type (which is
used to represent Kerberos principal names in the krb5 GSS mechanism)
without having to rototill the GSS krb5 mechanism nor the krb5 library,
and without affecting the encodings of HDB entries (which happen to use
the `Principal` type).
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