Change Pointer to void *

Change Pointer to void *

The comment for the Pointer type said 'XXX Pointer arithmetic is done
with this, so it can't be void * under "true" ANSI compilers.'.  This
has been fixed in the previous commit 756a4368932.  This now changes
the definition of the type from char * to void *, as envisaged by that
comment.

Extension code that relies on using Pointer for pointer arithmetic
will need to make changes similar to commit 756a4368932, but those
changes would be backward compatible.

Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/4154950a-47ae-4223-bd01-1235cc50e933%40eisentraut.org