Replace IteratorAggregate with Aggregate in ArrayInterface

[boesing]

Since there are internal PHP issues with native arrays regarding array key types (https://3v4l.org/Y2ld5), I was testing a bit and found out that we could work around this issue when we are actually refactoring the Array implementation to:

1. provide Iterator implementation (which then can provide proper types rather than exposing ArrayIterator)
2. we can also refactor Map implementation to persist keys and values in dedicated lists while having indexes pointing to the according value

The whole Map implementation should actually get refactored to avoid using array_* functions in most scenarios (esp. when it comes to array keys).

The return type of MapInterface#toNativeArray should also always have array-key, no matter of what the generic says. There is simply no guarantee that the array key is string and thus this is the best way to properly expose the weird PHP internal behavior which won't change too soon I guess.

I have some ideas but I might not work on this too soon, just want to dump my current thoughts.