My plan is to build a little interpreter for an esoteric programming
language as the most basic application of this package, before
embarking on some more ambitious applications.

Working my way towards a user-friendly introduction to the package.

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.

* Cargo.toml
* chain/Cargo.toml
* grammar/Cargo.toml: Bump version.

* ChangeLog: Add a short description of the bug fix introduced in this
  version.

This crate is responsible for the users to conveniently define their
own tokenizers.  Right now the user has to tokenize the input string
manually, which is not quite what is expected from a parser, from the
view point of users.

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.