What should we add?

Since #14006 merged we now have a representation of a circuit in the C API. However in that PR one component we didn't include was the function to add a delay to the circuit. This should be fairly straightforward but it wasn't included in that PR because there was a question about how we deal with units. At the time I was thinking we should discuss whether we want to support all the various SI prefixes or just take seconds and dt and save it for a follow-up. But also now since #14006 was originally opened we also now have support for stretches in Rust too, so we can potentially expose that via the C API too (but that can also be a separate task to not overly complicate this).