Add a simple example of working with stateful objects.

This BankAccount class contains state, mutable data representing the
balance of the account. Depositing and withdrawing modify the value of
the `private var bal` variable.
Add an example of functional code that uses vars.

You don't have to be all-vals to be functional, as we can see here. In
this case, we use a var as an implementation detail to cache the results
of some imaginary expensive operation.