add dynamic-graphs benchmarks

[ocramz]

dynamic-graphs is a new library for dynamic connectivity:

https://github.com/alang9/dynamic-graphs https://hackage.haskell.org/package/dynamic-graphs

(introductory blogpost: https://jaspervdj.be/posts/2019-01-11-dynamic-graphs.html )

It would be interesting to compare the performance of the reachability query.

[nobrakal]

Hi,

Seems very interesting, I will try to add this soon :)