Support pipelines, to declare subsections of action logic that can be referenced from many other places

[zachdaniel]

For example:

update :open do
  pipe_through [:change_state]
  change set_attribute(:state, :open)
end

update :close do
  pipe_through [:change_state], where: [expr(^actor(:role) == :super_user)]
  change set_attribute(:state, :open)
end

pipelines do
  pipeline :change_state do
    validate expr(condition), error_expr: expr(error(...))
    change atomic_update(:score, expr(score + 1))
    change switch(
      [{expr(condition1), change}]
      [{expr(condition1), change}]
      [{expr(condition1), change}]
    )
    change change2
    change change3
  end
end

This has many potential downstream use cases.