Don't assume size_t can carry pointer provenance and use uintptr_t
(identialy on all current platforms) instead.
Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Brooks Davis <brooks.davis@sri.com>
Closes#14131