Re: [streams] Requirement on promises equality (#266)

@domenic OK. So, it means that `rs.ready` should return the same instance until we're asked to replace `[[readyPromise]]` on e.g. `"readable"` to `"waiting"` transition?

How about the result of `cancel()` call on a `"closed"` or "`errored`" stream? It's now specified to return `[[closedPromise]]` than creating a new one resolved/rejected for efficiency. Do you think it also should be a requirement?

BTW, `writableStream.close()` can also be simplified to return its `[[closedPromise]]` when the stream is in the `"closed"` or `"errored"` state.

/cc @yutakahirano

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