Dep graph visualization

[mtzguido]

Some work in progress to make dependency graphs more useful. Mostly 1) distinguish between interface and implementation instead of just coallescing the modules, and 2) do not bring into the graph the implementation of modules if they are unneeded.

With this, the dependency graph for FStar.Tactics.V2.fst goes from:

To:

Which allows to distinguish fst/fsti (e.g. we are not depending on FStar.UInt32.fst). Marking as WIP since I ripped out the full cycle detection for this.