more complete handling of PDL conversion to R

This handles cases where the PDL is Empty (no dims and no value), a
scalar (no dims with a single value), and n-d arrays (dims and many
values).
add TODOs for conversion

Test more edges cases such as empty lists, vectors, etc.
convert Perl arrayref to R VECSXP

This recursively converts the contents of an arrayref to SEXPs and then
stuffs all of them in a VECSXP.