The `srfi/111` for boxes is missing

[Tuplanolla]

Racket already has boxing facilities in racket/base,
so they could easily be provided by srfi/111 as well.
The related srfi/161 and srfi/195 would probably take more effort.

[shawnw]

I have implementations of SRFI-111 and SRFI-195 in my extra-srfi-libs package for what it's worth.