Provide combinational depth insights

[sylefeb]

Related to but different from #5

It would be useful for the compiler to report a notion of computational depth, that is the longest combinational chain through dependencies (including through instances).

While this is far from actual hardware, it would give insights regarding the data paths and help visualize the flow of data and data overly long chains.

This relates to #5, since detecting combinational loops likely requires explicitly building the dependency chains.