[musl] Mallocng algorithm high-level overview

[Jack Bond-Preston <jack.bond-preston@arm.com>]

Hello,

I'm currently working on porting mallocng to a new architecture and
could use some assistance understanding the algorithm. From searching
the web, I couldn't seem to find any high-level overview of musl's
mallocng allocator (save for the readme at github/richfelker
/mallocng-draft, which is a little briefer than what I am looking
for). If any such description exists, I would much appreciate
being pointed towards it. If not, would anyone be able to explain
some of the details of the allocator?

Mostly I am interested in a more general high-level overview of how
the allocator works. There are also some specifics I am interested
in, if anyone is able to shine some light on these:
- The uses/purposes of the structures in meta.h. Particularly, meta
   and group, and the relation between the two.
- The general overview of in-band and out-of-band metadata, and how/
   when they are used.
- The purpose/meaning of the UNIT define in meta.h.
- Any assumptions about alignment/pointer size the allocator may
   make.

Thanks very much for your time, I appreciate the request is a bit
broad, but any information is appreciated. Please don't hesitate to
reach out for more information.

Cheers,
Jack