This replaces the dual ready + read() approach previously, which was derived from the epoll(7) + read(2) paradigm. In #253 we did a deep dive on real-world implementation strategies for byte streams, and discovered that the ready + read() model causes a conflict with the semantics we want for byte streams. Briefly, because some byte streams will demand to know the size of the buffer they must fill before doing any I/O (the fread(3) model), the byte stream method that reads into a given typed array must be asynchronous. If such byte streams are then to conform to the readable stream interface, with a no-argument read() method that is an auto-allocating version of the read-into method, then read() must also be async, across all readable streams.

This is a usability upgrade for consumers, in many cases. However, for non-byte streams, it potentially costs more microtasks when multiple chunks of data would be available synchronously. We are hopeful that even if current JS engine microtask queues are not as fast as they could be, they will improve over time until this overhead is negligible (which in theory it should be).

Alongside this change, we moved the read() method from the readable stream to the reader (now called "readable stream reader" instead of "exclusive stream reader"). This drastically simplifies the stream/reader interaction, and also allows the possibility of different types of readers which have different reading behavior---again, very helpful for byte streams. The usability impact is also positive, as it guides consumers toward using piping instead of directly reading chunks from the stream.

Note that read() promises fulfill with { value, done } instead of using an EOS sentinel value. This avoids a number of problems (see extensive discussion in #253), and also provides a mechanism by which readable byte streams can smuggle out "unused" buffers given to them, by using { value: zeroLengthViewOntoBuffer, done: true }.

Finally, the state property is now removed (from readable stream), since there is no longer a "waiting" vs. "readable" distinction.

This commit also adds some new infrastructure for _templated tests_, and ports some portion of the existing tests there. This is our solution for #217 and #264.

Note that we re-merged all related code into a single readable-stream.js file, as the setup with the three separate files (readable-stream.js, exclusive-stream-reader.js, and readable-stream-abstract-ops.js) was problematic in causing circular dependencies.

- Closes #253!
- Closes #266 by simplifying reader-related stuff, removing the problematic `ready` promise, and ensuring that cancel() always returns a new promise instead of reusing [[closedPromise]].
- Closes #264 by introducing templated tests.

See discussion in #297. This commit implements the following changes:

- Allow acquiring readers for closed or errored streams; they simply act closed or errored.
- Stop auto-releasing readers when streams close/error.
- Disallow canceling a stream that is locked to a reader (you should use the reader cancel).
- Piping from a closed or errored stream will close or abort the destination stream, instead of immediately failing the pipe.

See discussion in #297. pipeTo now returns a { finished, unpipe() } object, instead of just a promise.

See #297 (comment)

This reverts commit 0de787a. Per discussions in #297, we can punt on unpiping until we also figure out cancelable promises.

Conflicts:
	reference-implementation/lib/readable-stream.js

Per discussion in #297. We can always add it back later if we really want it, but it has become increasingly useless in the newer, reader-centric design.

The conservative thing to do is to not have this ability for now. Released readers are now indistinguishable from readers for closed streams.

As illustrated by the new tests, there were previously issues around auto-releasing not always giving the right result for errored or closed streams, especially with regard to allowing future readers to be acquired. This commit simplifies the logic and ensures consistency across all readers obtained from errored or closed streams.