Improve and clean-up JS variant of Tracer

These 7 changes clean up the handwritten JavaScript version of Tracer using suggestions by Alex Russell (@slightlyoff). Together they improve the performance by almost 10% but the only changes that really have a significant impact on the performance are these two:

  https://github.com/kasperl/ton80/commit/aae4b4a2f98e6fa735002ed55db86d45d6679ed4
  https://github.com/kasperl/ton80/commit/31820c28e962edcbf70458fcc6d3686510ad27a7

Comments or suggestions are most welcome!