Move the walker interfaces over to accepting Expressions instead of Cells.

All the walkers now use expressions internally, instead of Cells.  This allows
the parsing to be used into the Table class, avoiding having the walkers
needing the VariableCollection for a given table, simplifying their constructors
more.

Also clean up the imports, as they could be simplified due to this change.
Some imports may have not been necessary before this as well.