Streams3 - First Pass

[isaacs]

This addresses #5860.

Performance-wise it seems to be on-par with master, and I haven't gone through and worked out any trouble spots with the V8 profiling tools.

The commits are a mess, and will be squashed of course, but the best way to see the high level view of what this enables is to check out https://github.com/isaacs/node/blob/streams3/test/simple/test-stream3-pause-then-read.js. That test would switch a streams2 stream into old-mode, but with streams3, you hit s.pause() and it's right back where it was.

The other semantic change is that adding a readable event handler will not cause readable to be emitted until nextTick. Otherwise, stuff like this is problematic:

function MyReadable() {
  Readable.call(this);
  this.on('readable', function() {
    this.doSomeStuff();
  });
}

var r = new MyReadable();
// never gets called!  Already emitted in ctor
r.on('readable', function() {
  r.read();
});

All other semantics should be identical to master. API should be identical.

Still todo: docs.

Review/feedback please: @jclulow @bnoordhuis @indutny @Raynos @tjfontaine

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[trevnorris]

Looks like so in the doc output:

Calling the [pipe()][] method to send the...

[isaacs]

fixed.