Template data structures should implement iterators

[jpswinski]

The first//last functions for dictionary, list, ordering, and table should be made const. To do so, the first and last functions should return an iterator that maintains the state of the iteration.

For example:
for(iterator entry = dict.first(); entry.valid(); entry.next()) {...}

iter can hold a key and value pair of type T.

[jpswinski]

Iterators have been implemented for dictionaries and lists.

[jpswinski]

And now also for orderings.