Use the attribute API instead of the CustomData API, to correctly
handle anonymous attributes and simplify the code. One non-obvious
thing to note is that the type counts are recalculated by the "finish"
function of the `curve_type` attribute, so they don't need to be copied
explicitly. Also, the mutable attribute accessor cannot be an reference
if we want to give it an rvalue, which is convenient in this case.
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