Please consider providing Graph Reachability Matrix predicate

[LebedevRI]

It would have been rather useful to me when i was trying to write a model,
and i couldn't quite come up with anything good on a spot,
and thus after much pain ended up with:
https://github.com/LebedevRI/minizinc-graph-reachability-matrix-predicate.mzn

Unfortunately it ends up having O(n^3) complexity, as benchmark notes,
but that is only because connected() is itself O(n^2).
There may perhaps be a better formulation for all that,
but i'm not seeing it currently.
The results are already much better than i had hoped.

(There may be a standard set of inefficiencies in minizinc itself,
not sure how much i want to look into that given how it's developed and tested...)

[LebedevRI]

FYI i'm looking at a non-cubic implementation, hang on.