toml, monopam: follow loc Context move + plain Json.to_string
- toml error_to_loc: Loc.Error moved Context to top-level Loc.Context;
adjust to [Error.msg ~ctx:Context.empty ~meta:Meta.none].
- monopam: [open Jsont] -> [open Json.Codec] for codec definitions.
- monopam: Json.to_string returns plain string now, drop result patterns.
- monopam tests: the string-typed errors no longer need [Json.Error.to_string]
wraps at call sites.