Introduces promise streams

New concept: a promise for an iterable. Iterables include arrays,
objects with an "iterator" method that returns an iterator, iterators
(including generators and the new Queue).

The `next` method of a promise iterable may return a promise for the
next value, or the value may be a promise.

A new implementation of Queue provides an iterator interface.

`map`, `reduce`, `forEach`, and `buffer` are introduced for performing
various operations in parallel and serial, with input coming from any
iterable.  All of these support back-pressure, leaving values on the
input iteration until the number of concurrent promises have dropped
below some specified threshold.  All of these support notifications
for their internal buffer levels.

`all` has been reworked to accept a promise for any iterable.

This introduces a degree of conceptual unity around ES6 iterators.  As
such this drops support for SpiderMonkey's original iterators, and all
the corresponding shims.  This patch must only land in a
backward-incompatible release, probably v0.10.0.