Stepper protocol

[Eric Dean Tribble <tribble>]

A nice combination of hasValue, step, and value.  I originally
considered combining hasValue and step.  It unfortunately confuses the
abstraction.  I expect to create a Stepper already set to its first
value.  This is both cleaner and allows Steppers to be handed in to a
function and used in the iteration.  I would also like Steppers to
have the current value throughout each loop step.  Then they really
are much more like loop variables.  I'm tempted though...

dean