a few more testable file-system operations

Dependencies

• [2] ED4Z6ORC cleaner API for file-system access
• [*] 3QNOKBFM beginnings of a test harness
• [*] CUFW4EJL reorganize app.lua and its comments

Change contents

• edit in app.lua at line 293
  [5.4939]

  [5.4939]
  + read = function(self)

  + return App.filesystem[filename]

  + end,
• edit in app.lua at line 302
  [5.5001]

  [5.5001]
  + function App.mkdir(dirname)

  + -- nothing in test mode

  + end

  +

  + function App.remove(filename)

  + App.filesystem[filename] = nil

  + end

  +
• edit in app.lua at line 449
  [2.16535]

  [2.16535]
  + App.mkdir = nativefs.createDirectory

  + App.remove = nativefs.remove