Add Iterator#auto_rewind? and auto_rewind=

[makenowjust]

If Iterator#auto_rewind? returns true, the iterator is rewinded after Iterator#each automatically. Otherwise it returns false, the iterator is not rewinded.

Iterator#auto_rewind? default value is true, it is just breaking change.

This feature motivation is to be able to apply more Enumerable algorithm to Iterator. In current implementation, Iterator#each works only first time.

[asterite]

I'm not sure about this. I prefer an explicit rewind, otherwise it can be very confusing.

[makenowjust]

@asterite But we cannot use Iterator as Enumerable without attentions now...

[asterite]

@makenowjust Yes, because Iterator is not an Enumerable. You can use cycle to cycle.

[asterite]

(well, it's an Enumerable, but its elements are consumed, and once you do that, you need to invoke rewind. Auto-rewind is not good because it means it's almost implicit behaviour)