Add skippable iterators

[lemire]

Java's iterators can skip over values :

https://github.com/RoaringBitmap/RoaringBitmap/blob/master/roaringbitmap/src/main/java/org/roaringbitmap/PeekableIntIterator.java

[steveyen]

Looks like that link to the java iterator example is broken? Newer link: https://github.com/RoaringBitmap/RoaringBitmap/blob/master/roaringbitmap/src/main/java/org/roaringbitmap/PeekableIntIterator.java

[lemire]

Updated link.

[steveyen]

As an application usage example, the bleve full-text library would likely have use for a skip-ahead or advance-able iterator method. It currently has that brute force for-loop that calls HasNext/Next(), not really doing anything meaningful with the interim values...

https://github.com/blevesearch/bleve/blob/master/index/scorch/segment/zap/posting.go#L659

[lemire]

I have marked this as a priority.

It is not difficult to implement.