Fix ordering of queries by deduplicating indices.

[jerhard]

Due to merging, there were some types of queries had the same integer index. This PR deduplicates the indices of these query types.

[michael-schwarz]

It seems error-prone to do this ourselves: Could we maybe have a deriver for this, similar to what enum does here: https://github.com/ocaml-ppx/ppx_deriving

[sim642]

This order is the smaller of the worries there though. Duplicate indices just mean less short-circuiting in compare/equal and more collisions in hash.
The bigger problem is compare itself, where missing cases can cause real problems. The syntactic nature of PPXs means that it isn't really possible to derive compare for this type because the matching must happen on both type constructors (Any and the query) simultaneously but they're defined in different type declarations (because that's how constructors can only be defined).