atp, scitt: migrate to new Json API
- Drop jsont / jsont.bytesrw library deps in dune.inc; use json.
- Json.Codec.Value.null() / bool / number / string / array / mem / name ->
top-level Json.null / bool / etc. (these are value constructors, not codecs).
- Json.int / Json.int64 (as codec) -> Json.Codec.int / int64.
- Json.(list x) -> Json.Codec.(list x) where x is a codec.
- Json.object_' back to Json.object' (stray regex).
- xrpc_client: wrap Json.Error.t with Json.Error.to_string at the
Parse_error constructor boundary.