This changeset adds support for the iterable type for parallel() + series() + waterfall(). Instead of only accepting an array, each function now also accepts instances implementing Traversable such as Iterator and Generator instances.

This is a pure feature addition that does not break BC. With these changes applied, our API is now in line with the other promise APIs (see reactphp/promise#225).

This PR targets Async v4 as the minimum API version. The same changes have been merged for Async v3 already (#45).

The first commit highlights how this is essentially only a new type definition and a call to iterator_to_array(). The second commit then takes advantage of a more iterative approach that uses less memory and also supports consuming infinite iterators if the resulting promise settles. The test suite confirms this has 100% code coverage.

Refs #41
Builds on top of #11
Refs https://twitter.com/another_clue/status/1535652835521613824 (UML of iterators in PHP)