disable stringify operator

This seems to break the tests. I need to find out what is going on.
move data.frame conversion to its own function
move factor conversion to its own function
move charsxp and vecsxp conversions to their own functions
move the PDL conversions to their own functions
move R type info into its own file

Information about R's C-level types is moved from the PDL module to its
own file so that it can be useful elsewhere.