Rework GADT index types to get Lin compiling on trunk again

[jmid]

Fixes #559

ATM this changes two empty variant types in the interface, which is a bit annoying... 🤔

[jmid]

@OlivierNicole Do you recall why you chose empty variant types for constructible/deconstructible
and abstract types for combinable/noncombinable?

Rather than rewrite the interface, I guess another way around this is to silence the warnings with attributes on each pattern match that trunk will now deem potentially non-exhaustive. We know from OCaml 5.0.0-5.4.0 that they are in fact exhaustive. On the positive side this will be less invasive, however it also smells like more of a temporary workaround... 🤷

[jmid]

CI summary: all 50 workflows passed.

In the interest of unblocking trunk testing, I'll merge this.
We can revert and revise it later if we come up with a better solution.

[jmid]

CI summary for merge to main: all 51 workflows passed

[OlivierNicole]

@OlivierNicole Do you recall why you chose empty variant types for constructible/deconstructible and abstract types for combinable/noncombinable?

Sorry for the response delay, it looks like I missed the notification. No, I can't recall a good reason for this!