Add an RNA API function that gives an array of the normals for every control point.
The normals depend on the normal_mode attribute, which can currently be
minumum twist or Z-up, though more options are planned. Normals are currently
evaluated on the evaluated points and then sampled back to the control points.
Because that can be expensive, a normal mode that only does a first evaluation
on control points may be important

The function is intended to be used by Cycles, so it doesn't have to implement
the calculation of normals itself. They can be interpolated between control points
and normalized.

For best performance, the function should only be called once, since it does the
full calculation for every control point every time it is called.