Steps nest in their pipeline message; the steps list is gone

[dangra]

A pipeline is a top-level message with the pipeline option; its steps are the messages nested directly in it, in declaration order, which is the topology:

message ProvisionMachine {
  option (durable.v1.pipeline) = {id: "provision-machine" input: "..." output: "..."};

  message SelectHost      { option (durable.v1.step) = {id: "select-host/v1"}; string host_id = 1; }
  message ReserveCapacity { option (durable.v1.step) = {id: "reserve-capacity/v1" unwind: true}; string reservation_id = 1; }
  message CreateMachine   { option (durable.v1.step) = {id: "create-machine/v1"}; string machine_id = 1; }
}

• Nothing lists a step twice: inserting, reordering, or retiring a step is one edit, and the "declared but not listed / listed but not found / typo in the full name" errors cannot happen.
• A step belongs to one pipeline by construction; two pipelines in a package may name their steps alike.
• The step's Go type is protoc-gen-go's nested name, Pipeline_Step (*machinespb.ProvisionMachine_SelectHost, machinespb.ProvisionMachine_SelectHostStep); the handler method stays the short name. Input and output remain referenced top-level messages, since callers hold those types.
• Generation rejects a step outside a pipeline or nested deeper than directly in one, a non-step message nested in a pipeline, and a pipeline that is not top-level.
• PipelineOptions.steps is removed.

Examples, README (this includes #94's rewrite), tour, and spec 02 and 05 are in the nested form. Breaking for every proto: move the step messages inside the pipeline and drop the list; regenerate; rename state types to Pipeline_Step.

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