integer64 types do not dput wells with hexNumeric. Instead, we serialize and
then unserialize them cf truecluster/bit64#27 and store that as one expression per column
This makes the resulting data.frame awkward, but it will be recorded by dput in a
way that does get read back in faithfully (so long as the expression isn't quoted).
serialize_bit64(df_in)data.frame where each integer64 column is a serialized expression of itself
the data.frame to pre-serialize integer64 columns from