Remembering the number of curves of every type makes it fast to know whether processing specific to a single curve type has to be done. This information was accessed in quite a few places, so this should be an overall reduction in overhead for the new curves type. The cache is computed eagerly, in other words every time after changing the curve types. In order to reduce verbosity I added helper functions for some common ways to set the types. Differential Revision: https://developer.blender.org/D14732