This adds support for asn1_compile --decorate=... variation that causes
decoration of an ASN.1 SET/SEQUENCE type with a field of a non-ASN.1
type.
This means we can now have an ASN.1 type to represent a request that can
then have a "hidden" field -- hidden in that it is neither encoded nor
decoded. This field will be copied and freed when the decoration is of
an ASN.1 type or of a external, C type that comes with copy constructor
and destructor functions. Decoration with a `void *` field which is
neither copied nor freed is also supported.
We may end up using this to, for example, replace the `hdb_entry_ex`
type by decorating `HDB_entry` with a C type that points to the `HDB` in
which the entry was found or to which it should be written.
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