DEBT

* The on-disk library format used to be a tgz laid out in a particular way. This
  was called a `js_tar`, and there are still dangling references to this format.
  Namely the npm_to_jsar.py helper
* Internal rules and attributes should be underscored. This will break other
  rules that depend on the inner workings of these rules -- which is great.
* Now that this is all strung together, it would be a good use of time to try to
  fiddle with the binary format. For example, there may be a way to reduce I/O
  by writing (hard|sym) links to existing files -- the format need not be
  portable across machines.
* The `js_bundle` rules have no tests
* `js_library` data files are included in the jsar for compatibility reasons.
  This should be migrated to accumulate and expose to binary rules