ocaml-json: add value and codec module placeholders
Add one-line value.ml{i} and codec.ml{i} stubs, matching the shape of
ocaml-toml and ocaml-sexp. These are placeholders for the eventual
split of json.ml's monolithic contents into a value-centric module
(AST + accessors) and a codec-combinator module; for now json.ml
still carries both.