ESS 0.55.0

Added

• ess generate cli admits a CLI result field typed Json, alone or under Optional, List or
  a String-keyed Map. The compiled plan carries a json shape; the generated adapter writes
  the value as JSON rather than JSON text, and its result check accepts any JSON value there and
  refuses a reply outside the declared shape as cli_result. An input or error field carrying
  Json is refused, naming result fields as where it is admitted; a reader without the json
  shape refuses such a plan as an unknown variant. Generated CLI packages carry new src/wire.rs
  bytes, so ess generate cli --check reports drift until they are regenerated
  (#468).

Changed

• A {generated: true} payload value of type Optional<T> is now supplied by the
  implementation: generated Rust and Go behaviours read it from a new context port method,
  generate_optional_<t> returning Option<T> (try_generate_optional_<t> on TryContext) and
  GenerateOptional<T> returning *T (TryGenerateOptional<T> on FallibleContext), instead of
  always filling it absent. A context that implements the port must add the method. The generated
  demonstration context answers it absent. An event field nothing sets stays absent and asks no
  port (#467).