Similar to 8d0920ec6d.
In a test case with 8 million points and 3 attributes, I observed
around a 9x performance improvement, from 1.8s to 0.2s to copy
the data from Blender to Cycles. For some attribute types, using
implicit sharing could remove the need to copy entirely, but removing
the overhead from the RNA API makes sense anyway.
aef0e72e5a