Make dumpast less verbose

[n-osborne]

The dumpast is relying on [@@deriving show], which is nice as we didn't have to implement the printer. But this leads to a very verbose output compared to ocamlc -dtypedtree or ocamlc -dparsetree. Maybe we could benefit from a less verbose and more on point output. What are the current use cases of this command?

In the Gospel repo, the command is used in cram tests.

On my side, I use it to have a concrete example to explore of what I am dealing with when translating a particular pattern of specifications to OCaml in Ortac.