To compute the difference between two addresses, cast the pointers

To compute the difference between two addresses, cast the pointers
holding those addresses to "void *" and then to "char *" (so we don't
get warnings from casting directly to "char *" or errors from
subtracting two "void *"s), and subtract them, rather than casting the
pointers to an integral type possibly shorter than the pointers (to
avoid warnings and to avoid the admittedly-infinitesimal chance that the
two pointers don't differ in the bits that fit into the integral type).

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