make sure that the input to `convert_r_to_perl` is always an `R::Sexp`.

Later on, it might be worth looking at Type::Tiny and parameter
checking.
Simpler conversion comparison for Perl and PDL data

Make the comparison simpler by looping over the `pdl_data` and
`perl_data` keys and using the blessed type to figure out which
comparison function to use.
add `convert_perl_to_r` function stubs to DataFrame and Factor conversion