On Mac, size_t is a distinct type from uint64_t, and some
types (in wallet cache as well as cold/hot wallet transfer
data) use pairs/containers with size_t as fields. Mac would
save those as full size, while other platforms would save
them as varints. Might apply to other platforms where the
types are distinct.
There's a nasty hack for backward compatibility, which can
go after a couple forks.
4a9ae3eb8b