parse-wormedit: declare hardcoded bytes in unpackf template

Recognise a new template value '=$N' (where $N is any number) in
Parse::Binary::Nested to insert a hardcoded value instead of reading it.
This allows object structures of v90 files without type declarations to
be returned in the same format as newer versions.  Still requires some
custom merging of the multiple arrays afterwards, but this may be solved
later on.