Add Bitmap.NextAbsentValue

[ghost]

Java implementation has nextAbsentValue method. Go implementation doesn't. Could you please add it?

https://www.javadoc.io/doc/org.roaringbitmap/RoaringBitmap

public long nextAbsentValue​(int fromValue)

Returns the first absent value equal to or larger than
the provided value (interpreted as an unsigned integer).
It is not necessarily a computationally effective way
to iterate through the values.

Parameters:
    fromValue - the lower bound (inclusive)
Returns:
    the smallest absent value larger than or equal to
    the specified value.

[lemire]

That's a good issue. Do you want to try it out?

[lemire]

A PR is invited.

[ghost]

Looks like a lot of work. I don't think I need this method anymore, nor do I need the companion functions previousAbsentValue, nextValue, previousValue.

[lemire]

Looks like a lot of work.

It is not too bad, but it requires a minimum of care and a few hours.

[bearrito]

@lemire Once the validation stuff gets in I'll grab this. I think have a good enough grasp on the data structures to do this.