Re: [streams] Change the model for ReadableStream to have async read() (#288)

OK so.

- `rbs.readInto(ab, offset, size)` will use `ArrayBuffer.transfer` to create a new array buffer, `rbsAB`, whose contents are transferred from `ab`. Thus `ab` is detached. Eventually the returned promise is fulfilled with `ab2` (instead of bytesRead). That is pretty nice! Except for the issue mentioned in https://github.com/whatwg/streams/issues/253#issuecomment-75790533.

- `const p1 = rbs.readInto(ab, size); const reader = rbs.getReader(); const p2 = reader.readInto(ab2, size2)` will use `ArrayBuffer.transfer` to create a new array buffer, `rbsAB`, whose contents are transferred from `ab`. `ab` is now detached. Eventually the read finishes and `rbsAB` has bytes in it. Then, under 1, `p1` is rejected, whereas under 3, it remains pending. In both cases, do we fulfill `p2` with `rbsAB`? Do we just ignore `ab2`?

Hmm. Now that we have this detaching-returning-different-buffers thing, maybe async readInto is not the right model, and feedArrayBuffer or setAllocator would make more sense. It feels weird to supply an argument to `readInto` whose backing memory is used, while the actual array buffer in question gets detached and is no longer useful. Even if we changed the name (`readInto` no longer makes sense, but e.g. `readBytes` is fine) it's still an awkward API.

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