Optimize data clamping in spatial view

[emilk]

What

We do a lot of "clamped joins", where the last element of a component is repeated to match the number of instances.
The code for this was neither ergonomic, nor performant. Now it should be better at both.

In particular, complex iterators like .chain(…).take(…) are often slow and difficult for the optimizer to see through.

Read the commit messages!

This shaves off another 0.5-1 ms off of OPF, out of ~4ms for the inner Points3D visualizer.

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