message: Complete `tezedge_context_empty`.
diff: @@ -157,15 +157,19 @@ ocaml_export! {
  result.to_ocaml(rt)
  }
  - // TODO: implement
  fn tezedge_context_empty(
  rt,
  - _unit: OCamlRef<()>,
  - ) {
  - OCaml::unit()
  + index: OCamlRef<TezedgeIndex>,
  + ) -> OCaml<TezedgeContext> {
  + let index_ptr: OCamlToRustPointer<TezedgeIndex> = index.to_rust(rt);
  + let index = index_ptr.as_ref();
  + let empty_context = TezedgeContext::new(index.clone(), None, None);
  + let result = OCamlToRustPointer::alloc_custom(rt, empty_context);
  +
  + result.to_ocaml(rt)
  }
  - // OCaml = val dir_mem : context -> key -> bool Lwt.t
  + // OCaml = val mem_tree : context -> key -> bool Lwt.t
  fn tezedge_context_mem_tree(
  rt,
  context: OCamlRef<TezedgeContext>,
  @@ -381,7 +385,7 @@ ocaml_export! {
  result.to_ocaml(rt)
  }
  - // OCaml = val dir_mem : tree -> key -> bool Lwt.t
  + // OCaml = val mem_tree : tree -> key -> bool Lwt.t
  fn tezedge_tree_mem_tree(
  rt,
  tree: OCamlRef<WorkingTreeRc>,
  