ocaml-scitt: enable MDX on lib/atp/scitt_atp.mli, fix broken doc example
Run mdx on lib/atp/scitt_atp.mli so the {[ ... ]} odoc block now
type-checks. The example called Heap_b.v -- a fictional shorthand the
test file constructs by hand via Irmin.Heap.Make over a custom
BACKEND. That backend is ~15 lines of test scaffolding that does not
belong inline in the .mli.
Replaced with a one-line `Irmin.Heap.of_list ~equal:Atp.Cid.equal []`
that gives a heap of the right type without the backend boilerplate;
this is enough to typecheck the Scitt_atp.v construction the doc
demonstrates.