Add feature flags for various Insim versions

[theangryangel]

Similar to how the k8s_openapi crate works, since Insim has historically been additive in its packet definitions, it seems reasonable to bend feature flags to our will.

This is a holding issue for that work.

[theangryangel]

The inkwell llvm_versions macro is also quite a nice implementation. More so I think than the k8s_openapi crate.