Change some Datum to void * for opaque pass-through pointer

I developed this patch as part of the Datum-as-struct semi-proposal [0]. 
  I think it can be useful independent of that.

Here, Datum was used to pass around an opaque pointer between a group of 
functions.  But one might as well use void * for that; the use of Datum 
doesn't achieve anything here and is just distracting.


[0]: 
https://www.postgresql.org/message-id/flat/8246d7ff-f4b7-4363-913e-827dadfeb145%40eisentraut.org