Don't rely on pointer arithmetic with Pointer type

Don't rely on pointer arithmetic with Pointer type

The comment for the Pointer type says 'XXX Pointer arithmetic is done
with this, so it can't be void * under "true" ANSI compilers.'.  This
fixes that.  Change from Pointer to use char * explicitly where
pointer arithmetic is needed.  This makes the meaning of the code
clearer locally and removes a dependency on the actual definition of
the Pointer type.  (The definition of the Pointer type is not changed
in this commit.)

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