Delete useless /meta/ constructor of the Tree format
Delete useless /meta/ constructor of the Tree format

The description of the Tree format is represented by an [encore]'s value
[format] (and exposed by the interface). This patch is a translation from
the /meta-syntax/ to [encore]'s combinators.

mirage/ocaml-git,mirage/ocaml-git,mirage/ocaml-git