The purpose of this change is to share this trait with other crates,
such as the forth-coming "semiring" crate that will be responsible for
handling some simple semiring operations as well as the querying, in
my plans.

Previously some incorrect forest nodes will be used for planting new
nodes.  I cannot fix the root cause of their presence in the
chain-rule machine.  But I can ignore them when they are encountered.

Of course I would like to really prevent them from existing, but still
cannot figure out how.

Now the binding part is finished.

What remains is a bug encountered when planting a fragment to the
forest which intersects a packed node, which would lead to invalid
forests.  This will also cause problem when planting a packed
fragment, but until now my testing grammars do not produce packed
fragments, so this problem is not encountered yet.

I am still figuring out efficient ways to solve this problem.

* chain/src/default.rs:
* chain/src/lib.rs: Add a parameter that controls whether or not the
chain-rule machine computes the item derivation forest as well.
Sometimes we only need to recognize whether an input belongs to the
grammar, but do not care about the derivations.  This parameter can
speed up the machine in that case.

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.

Previously there was a minor bug: if the chain-rule machine ended in a
node without children, which node should be accepting because of edges
that have no children and hence were ignored, then since the node has
no children, it would be regarded as not accepting.  Now this issue is
fixed by introducting real or imaginary edges, where an imaginary edge
is used to determine the acceptance of nodes without chidlren.

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.