Copy Ceed Object References

[jeremylt]

This adds reference copying to libCEED. This creates a second, separate reference to the same Ceed object that can (and should) be independently destroyed.

@knepley, I think this will address your segfault. @YohannDudouit, this might also be of interest for MFEM.

[jeremylt]

Note: I can't reproduce the CI issue locally yet. I'm investigating - it might be a cargo_tarpaulin issue?

[jeremylt]

This crate isn't building correctly in CI: https://github.com/KyleMayes/clang-sys
but I'm not sure why. It doesn't have anything to do with us though

[jeremylt]

I don't know how I fixed it, but I fixed CI. Ready to merge.

[jedbrown]

All looks fine. Would using "reference" as a verb, as in CeedReference(Ceed ceed), with CeedReferenceCopy(Ceed src, Ceed *dest) or CeedReferenceShare(Ceed src, Ceed *dest) be better for autocomplete?
PETSc does not have a "copy" function like this, but it could be used most places where we currently spell out the operations.

[jeremylt]

I updated the function names. A fix is in for rust-lang/rust on what I think the Rust issue is, but I don't know when it will merge. We should be ok to just merge whenever you're ready @jedbrown

[YohannDudouit]

@jeremylt Was there a specific reason for you to think this could be useful in MFEM?
Otherwise this looks good to me.

[jeremylt]

I don't have a current specific use case in mind - it just seems like something that might be useful in the future

[jeremylt]

Rust is happy, but GitHub is confused. This is good but no green button for some reason