Build sfi index in d4 files?
Hi @brentp, would you be open to this? It seems like the d4 bindings don't have an api for building the index, but it should be fairly simple to do, its available to the python bindings: https://github.com/38/d4-format/blob/480e0a36a453fd3ec40b6768458f8892e6f36510/pyd4/src/builder.rs#L129-L144. 

I'd be happy to work on implementing this, but want to check if you're open to it first. Would require changes in d4-nim too presumably