These compute size classes and indices similarly to size2index(),
index2size() and s2u(), respectively, but using the subset of size
classes that are multiples of the page size. Note that pszind_t and
szind_t are not interchangeable.
This fixes compilation warnings regarding integer overflow that were
introduced by 0c516a00c4cb28cff55ce0995f756b5aae074c9e (Make *allocx()
size class overflow behavior defined.).
