Consider supporting dynamic loading of the C++ library

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That would allow updating the C++ library without having to relink the Rust crate (and then, transitively, all dependencies which depend on the Rust crate)

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I realized this would be a significant change than I first was thinking, since we'd have to decide which features are available dynamically rather than by checking the version at build time. Might still be worth it though.

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Hmm also we probably would still need to support static linking for envs that prefer it or where shared libs are unavailable...