C++ support for stepper generation
Constructors for dense steppers
Full support for dense stepper creation.

Can't be used yet, but test-stepper.R passes and support functions
are all there.
Preliminary dense output support.

There are some sketchy things to deal with still here,
including one crash, some odd results, one stepper that
failed to run.  I'd regard the dense steppers interface
as very experimental.