Make execute_context and js_execute_context work

execute_context.js:
* Transition from the old wam.Event API to AxiomEvent (which has an explicit
  fire() method.)

js_directory.js:
* Rename linkEntry->link(), linkFileSystem->mount()
* Add install() which turns a {name: callback, ...} map into JsExecutables and
  links them into the directory.

js_executable.js:
* Pass the ExecuteContextBinding to the callback as the first param, since
  that's where the most useful functionality lives.  The JsExecuteContext is
  available as the second param in case the callee needs the "upcast" version
  for some reason.

js_file_system.js:
* Remove debug log.
* Change linkFileSystem() callsite to mount().