I might change this convention again in the future, particularly once
someone wants to fund C89 conformance, but let's stay consistent for
now.
Part of P0264, funded by [Anonymous] and Blue Bolt.
I could have equally argued the opposite way, and in favor of `sgm` and
`ofs`, but I've also been using `seg` and `off` more prevalently
throughout the years.
Part of P0245, funded by [Anonymous], Blue Bolt, Ember2528, and Yanga.
Getting us completely macro-free there… even though it did require a
separate version of those functions if the ID is a pointer.
Part of P0134, funded by [Anonymous].