This provides macros for managing intrusive doubly linked lists.
There are many ways how to do those in a "generic" way in C. For example
Solaris style lists are pretty nice:
https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/sys/list.hhttps://github.com/illumos/illumos-gate/blob/master/usr/src/common/list/list.c
I even have an independent implementation of this, which could be ISC
licensed. But I think it's easier to vomit ~100 lines of preprocessor
garbage, which has a lower footprint, and I think it wins slightly on
the side of type safety, simplicity, and ease of use, even if it doesn't
look as magically nice.
