incorporate suggestions by Stephan

``new_user_whatsit()`` (alias: ``new_user_whatsit_factory()``) returns a node generator, not the bare node.

new test file:

   \input luatexbase.sty
   \directlua{
   
   %% we create a couple whatsit generators for testing
   %% first two are unassociated (packageless)
       local some_whatsit,       id1 = luatexbase.new_user_whatsit"foo"
       local some_other_whatsit, id2 = luatexbase.new_user_whatsit"neato"
       print(id1, some_whatsit, some_whatsit())
       print(id2, some_other_whatsit, some_other_whatsit())
   
   %% this uses the recommended way of registering with a package name (“bar”)
       local some_package_whatsit, id3 = luatexbase.new_user_whatsit("foo", "bar")
       print(id3, some_package_whatsit, some_package_whatsit())
   
   %% this does the same, but requests the id only
       local id4 = luatexbase.new_user_whatsit_id("mywhatsit", "bar")
       print(id4)
   
   %% registering the same name/package combo again triggers a warning
       local id5 = luatexbase.new_user_whatsit_id("mywhatsit", "bar")
       print(id5, id4 == id5)
   
   %% id without a package (unassociated)
       local id6 = luatexbase.new_user_whatsit_id"baz"
       print(id6) %% == 5
   
   %% get the name and package the whatsit with number “3” was registered for
   %% (should be “foo” and “bar” in this example)
       local thirdname, thirdpackage = luatexbase.get_user_whatsit_name(id3)
       print("name of 3rd:", thirdname, "from package:", thirdpackage)
   
   %% also, same is possible for the node itself ...
       local whatsit_instance = some_whatsit()
       local what_name, package_name = luatexbase.get_user_whatsit_name(whatsit_instance)
       print(whatsit_instance, "->", what_name, package_name)
   
   %% ... and the generator function:
       local what_name, package_name = luatexbase.get_user_whatsit_name(some_whatsit)
       print(some_whatsit, "->", what_name, package_name)
   
   %% unknown whatsit ids cause a warning
       local _, _ = luatexbase.get_user_whatsit_name(42)
   
   %% get the id of a registered whatsit
       local known_id = luatexbase.get_user_whatsit_id"neato"
       print("id of “neato”:", known_id)
   
   %% anonymous whatsits have neither a package nor a name associated with them
   %% they are intended for quick hacks and stackexchange answers, not for
   %% packages
       local anonymous_whatsit       = luatexbase.new_user_whatsit()
       local anonymous_whatsit_too   = luatexbase.new_user_whatsit()
   
   %% lastly, there’s is some basic introspection functionality
   %%    a) print all known user whatsits to the terminal
       luatexbase.dump_registered_whatsits()
   %%    b) print whatsits belonging to a package
       luatexbase.dump_registered_whatsits"bar"
   }
   \bye