Procedural macro backing alloc_once!.
alloc_once!(v1, v2, ...) packs the values into a single tuple, allocates
that tuple once inside a freshly created bump arena, and returns an
alloc_once::AllocOnce guard that owns the arena. All bumpalo details are
hidden: the expansion only names paths under ::alloc_once, so callers do
not need a bumpalo dependency of their own.