breaking: end on read (part 1)

[jeswr]

resolves #25
resolves #35

Part of #43 without any lint or limit changes

[coveralls]

Coverage remained the same at 100.0% when pulling 7cb8a1d on jeswr:breaking/end-on-read-call into 7a089e3 on RubenVerborgh:main.

[jeswr]

I feel the changes here could do with a little explanation:

In short - since end is now triggered by calling .read, we need to make sure that we now always have triggered at least one .read call before checking if the iterator is .done.

At the same time I added the ability to add data from multiple consecutive Transformers in the queue so long as they have data ready; this can reduce excessive calls to _read if the transformers in the queue only produce a small number of elements each.

[jeswr]

@RubenVerborgh this is ready for review - apparently I don't have permissions to set you as a reviewer over here ...

[RubenVerborgh]

Copying some test code by @jeswr from #75

        // TODO: Re-enable this section check for once https://github.com/RubenVerborgh/AsyncIterator/pull/45
        // is merged in order to check that destruction occurs immediately rather than over several ticks
        // it('should destroy when closed before being read after map', async () => {
        //   iterator.map(x => x).close();
        //   // TODO: Re-enable this same-tick check once https://github.com/RubenVerborgh/AsyncIterator/pull/45
        //   // is merged
        //   // iterator.destroyed.should.be.true;

        //   // Wait a tick
        //   await Promise.resolve();
        //   iterator.destroyed.should.be.true;
        //   console.log('b', iterator._state, iterator.open, iterator.closed, iterator.ended, iterator.destroyed)
        // });

        // it('should destroy when closed before being read after map then filter', async () => {
        //   const iter = iterator.map(x => x);
        //   iter.filter(x => true).close();

        //   // TODO: Re-enable this same-tick check once https://github.com/RubenVerborgh/AsyncIterator/pull/45
        //   // is merged
        //   // iterator.destroyed.should.be.true;
        //   // it.destroyed.should.be.true;

        //   // Wait a tick
        //   await Promise.resolve();
        //   iterator.destroyed.should.be.true;
        //   iter.destroyed.should.be.true;
        // });

[jeswr]

Note to self: whenever I revisit this PR, I should add some more tests, and review the code, to make sure that error events are never emitted before the iterator is started.