Provide future-proof record constructors

[dsheets]

Either default records (less control) or constructor functions (more control) would allow users to write future-proofed code that creates GitHub record types. This would let us move the GitHub record types off the breaking change list. See ocaml-opam/opam-publish#45 for the original suggestion by @AltGr.

[kit-ty-kate]

I agree. I encountered the same issue while reviewing #280. However, i've tried atdgen -v but it seems like this option might have an issue ahrefs/atd#433, so i'm hoping to be able to provide a new Github_v module which would provide future-proof constructors, once this is fixed.