Create the `Curves` data-block directly, instead of using the conversion
from `CurveEval`. This means that the `tilt` and `radius` attributes
don't need to be allocated. The old behavior is kept by using the right
defaults in the conversion to `CurveEval` later.
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I mostly did this as a simple test of the new code. At least it will remove
the overhead from the type conversion for these nodes.