First-class representation of Item/ItemID

[dabrahams]

First of all Chart.Item should be renamed Chart.ItemID, since multiple entries with the same ID
actually constitute an item.

Then, perhaps Entry should be renamed Cause or Causation or Derivation.

Then Item should be a first-class type wrapping a Chart.Entries.SubSequence, and having a sequence of Derivations or at least MainstemIndices.

Then there should be a first-class representation for the sequence of Items in an Earleme having a given
transition symbol.

All of this is currently ad-hoc in the type system.