Upgrade small_vector from chobo to itlib

Paul Harrisrequested to merge
paulharris/diy:fix-small_vector.hpp into master

The old small_vector.hpp is not c++20 compliant.

Chobolabs repo is archived, and the code is now maintained by iboB old: https://github.com/Chobolabs/chobo-shl new: https://github.com/iboB/itlib/

I've done this for VTK (PR pending), compiles ok, but I haven't checked if these changes compile in the context here.

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