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.

Not bug deals but adjustments of details.

* chain/src/item/default/splone.rs: A function for finding the node to
  plant used to ignore the condition that the children should match.
  Now this is fixed.

* chain/src/item/mod.rs: Some function and enum are needed across
  crates.

* chain/src/atom/default.rs: Previously the nullibility of virtual
  nodes is determined by the nullibility of all its out-going edges.
  This is now determined only by the nullibility of out-going edges
  that are not "left-linearly expanded".  This is the correct
  approach.

* chain/src/default.rs: This is useful for debugging the chain-rule
  machine.

Some bugs are fixed:

1. If a non-terminal expansion can be reduced immediately, previously
an extra node would be created that had no parents.  Now this strange
behaviour is corrected.

2. When performing reductions, a leaf non-terminal node would
previously be regarded as completed.  Now we will first try to
complete that node, and then determine if the completion is
successful, and finally determine the completedness according to the
result.

Of course some more tests are still pending, before I can confirm that
no more bugs lurk around.

Adjust the codes slightly.

Also add a plan to implement the context-free memoization.

* chain/src/item/default/printer.lldb:
* chain/src/item/default/printer.py: These are for experimenting with
  debugger supports.