* grammar/src/label.rs (set_end_option): This function replaces the
  old function `open_end`, as this new function is more general than
  the old one, and there is no specific situation where we only need
  to open the end of a node without the need to close the node in an
  `if` statement.

I decide to adopt a new approach of recording and updating item
derivation forests.  Since this affects a lot of things, I decide to
commit before the refactor, so that I can create a branch for that
refactor.

I need more than the ability to clone nodes: I also need to split the
nodes.  Now this seems to be correctly added.

It seems to be complete now, but still awaits more tests to see where
the errors are, which should be plenty, haha.

Now the forest can detect if a node is packed or cloned, and correctly
clones a node in those circumstances.  But it still needs to be
tested.

I have an ostensibly working prototype now.

Further tests are needed to make sure that the algorithm meets the
time complexity requirement, though.