Using the new index mask implementation, things can be a bit simpler.
It's also simpler to use `complement` instead of `to_ranges_invert`,
which just made everything less standard. Also create the new curve
offsets in place instead of copying, and use implicit sharing to share
attributes when no curves were deleted.
With a version of `IndexMask::complement()` optmized locally, I observed
the following speedups with a 1.2 million point curve system:
- Delete points: 29 FPS -> 45 FPS
- Delete curves: 48 FPS -> 49 FPS
- Delete tip points: 25 FPS -> 32 FPS
Also add a method to apply the "gather" function to all attributes,
mostly as a continued experiment of consolidating attribute propagation.
This can be used more elsewhere in the future.
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