v1.5.0

Typed outcomes (decision D23). Opt-in with typed_outcomes: an action that
doesn't turn it on runs as before, except that a code whose fails or skips
declaration carries a block (new syntax) has its output validated.

Added

• typed_outcomes turns typed outcomes on for an action and its
  descendants (typed_outcomes enabled: false turns them off for a subclass);
  typed_outcomes? reads it.
• A block on fails / skips declares the output a result with that code
  carries, in the output DSL. failure_output_schema(code) and
  skip_output_schema(code) read it back, and describe shows its fields.
  Such output is built from the call's keywords alone, never from instance
  variables, and validated: output that breaks the schema ends the run as
  invalid_output. The block also ends instance-variable capture for that
  code, typed outcomes or not.
• output: on fail / fail! / fail_with / fail_with!, under typed
  outcomes only: the failure's output, a Hash or a FieldStruct
  (ArgumentError otherwise). Without them output: on fail is still an
  error, and fail_with refuses it. Under typed outcomes, output for a code
  with no schema raises DefinitionError, naming the code.
• Per-action result classes. Under typed outcomes .run returns the
  action's own Success, Failure or Skipped, each a subclass of the gem
  class (or of the parent action's own). A nested failure becomes the parent's
  own Failure, judged by the parent's schema for its code; a parent without
  typed outcomes gets a plain Actionable::Failure. The RSpec stub helpers
  return them too, with output shaped through the code's schema (or empty
  without one) as a real run would.
• RBS: Actionable::RBS.generate types a typed action's .run as
  (Success | Skipped | Failure), with each code and output narrowed and
  the output's readers declared on each class. A host's own result subclass,
  which .run still returns as it is, is not described: a known gap.

Changed

• Under typed outcomes, a skip or failure whose code has no schema carries an
  empty output, where it used to carry a best-effort capture into the success
  schema. The gem's own invalid_input / invalid_output keep theirs.
• Under typed outcomes, results compare by class: Act::Success is not ==
  to an Actionable::Success with the same output, and the stub helpers
  return a new object of the action's own class. Specs that compare results
  with == need to compare against the action's class.