[WIP] feature/countable stream iterator

[basz]

@prolic This is what I came up with. As always naming is an issue, php isn't consistent when it comes to interface naming. IMO 'Iterator' should have been called 'Iterable'.

This PR;

• introduces an StreamIterator namespace.
• provides an abstract StreamIterator class. It implements Iterator and Countable.
• implements an EmptyStreamIterator and an ArrayStreamIterator from the abstract.
• changes the EventStore implementations load and loadReverse methods to return a StreamIterator implementation. It does this without changing the return type of Iterator. It does not seem not possible to change the Prooph\EventStore\ReadOnlyEventStore interface to have a return type of StreamIterator without a BC break. :-( I do believe it is a minor BC as 'probably' will only break tests that use ArrayIterator or EmptyIterator (like you did).

Q: Is there a reason you are utilizing the EmptyIterator over an empty ArrayIterator?

Although I also provided an EmptyStreamIterator (you were using EmptyIterator) I think an ArrayStreamIterator (with an empty array) would be just as good. Both are countable now, but it might be a possibly BC break. Probably anyone currently checking for $i instanceof EmptyIterator somewhere.

[coveralls]

Coverage increased (+0.0007%) to 99.007% when pulling c23e10d on basz:origin/feature/countable-stream-iterator into a6389e3 on prooph:master.

[prolic]

looks good.

[prolic]

I think this should be an interface extending the other two, as there is not any implementation here.

[prolic]

The iterator instance returned is an implementation detail. We guarantee only the iterator interface as return type, therefore this is not a bc.