generating files has bitrotted

[StefanKarpinski]

See #95 (comment) and #99: in short, we've lost the ability to generate the wrapper code for libcurl. Would be great to

1. Artifactize the libcurl headers, possibly by including them in the JLL and
2. Be able to generate code from that in a reproducible way
3. Maybe add a CI step to make sure that what's committed matches what's generated afresh.