[rstream-dot] refactor to use @thi.ng/dot

[postspectacular]

The new @thi.ng/dot package is more general and offers better support for various Graphviz options, incl. node record shapes with labeled inputs/outputs. rstream-dot should be rewritten using these features.

• replace DotOpts with GraphAttribs from @thi.ng/dot
• update walk process with special support for StreamSync, StreamMerge and possibly PubSub to make use of multi-input/output nodes for cleaner visualizations

The issue with that 2nd point is that StreamSync and StreamMerge use internal/hidden subscriptions, which in the graphviz model will need to be translated as ports of the owner node. This breaks the 1:1 relationship between actual graph topology & the visualization, but will be easier and less confusing to read, however makes the whole graph traversal and edge collection harder...

walk() might have to be rewritten to use breadth-first (rather than the current depth-first) traversal, in order to correctly map StreamSync/Merge nodes in the new/proposed model.