Tracevis rewrite

Highlights:

* We now have a test coverage report for the library code in tracevis
* Improved test coverage
* Simplified many of the transforms by keeping everything in graph form
  and applying transforms (e.g. for filtering)
* Removed dig.js - it's been EOL'd
* Added graphlib.js - replaces dig.js
* Added graphlib-dot.js - used for rendering graphviz output
* Renamed trace.html to index.html
* Tracevis now builds its package itself

@chikit @jodzga