simple stringification of R::Sexp objects

currently this just calls `R_PrintValue()` and returns an empty string.
In the future, this should still call `R_PrintValue()`, but the console
output should be collected in a filehandle (e.g, opening a scalar to
write to).
wip: first pass at handling function calls

This has a hack that treats the arguments as Perl integers. This will be
removed in the future when all the arguments are R::Sexp objects.