Upstreaming Facebook extensions

[Augie Fackler]

> On Nov 13, 2017, at 22:12, Jun Wu <quark at fb.com> wrote:
> 
> For absorb, it needs to migrate away from using Cython, and has a pure
> implementation. Unfortunately I cannot prioritize this work for now.

FWIW I still intend to get to this work, as the algorithms side of it interests me pretty greatly.

(But if someone else wants to pick it up by all means please do so, but ping me so I don't get started on it at some point.)