Track 16-bit timer values

Move to 16-bit quanta instead of 64-bit time-of-day trackers.  This will
make the atmel AVR happier and faster, and it also moves in the direction
of supporting a linear velocity profile array in the future.

Although, it is beginning to look like we might need to store acceleration
values instead of velocity ones, that will actually make it easier for us
to calculate the velocity curve over time.
Group movement vars into a structure

For now it is a singleton global structure.  Later we will use it
to enable tracking two movements at once so they can be overlapped.
More type-width reduction

Getting closer to AVR-friendly, and still accurate.
Trivial noise reduction
Convert to acceleration-profile

I've been calling this a velocity profile all this time, but we
might as well use acceleration instead.  The only velocity we really
work with is our accumulated velocity and vmax.  All the rest of the
math works with acceleration.

Switching this way has a pleasing effect on the code.  It causes
us not to subtract v(Td-Ts) from our current velocity but instead to
add the negative acceleration instead.  This also fits better with
our phase approach we will need for the later acceleration profile.
I think.
Cleanup and comments