feat(iter): Add next_many() for batch iteration

[Zeutschler]

Summary

Add next_many(&mut self, dst: &mut [u32]) -> usize method to Iter and IntoIter for efficient batch extraction of bitmap values into a user-provided buffer.

Motivation

When iterating over large bitmaps, calling next() repeatedly incurs significant per-element overhead. The next_many() method extracts multiple values at once, enabling:

• Reduced function call overhead
• Better cache locality with contiguous buffer writes
• ILP-friendly processing of batched results

This API mirrors the next_many() method available in CRoaring and the Go implementation of RoaringBitmap.

Performance

Benchmark	next()	next_many()	Speedup
Dense (1M values, bitmap storage)	19.02ms	6.16ms	3.09x
Sparse (10K values, array storage)	1.67ms	159.58µs	10.46x

API

let mut iter = bitmap.iter();
let mut buf = [0u32; 1024];
loop {
    let count = iter.next_many(&mut buf);
    if count == 0 { break; }
    // Process buf[..count]
}

Returns the number of values written to dst. Returns 0 when the iterator is exhausted.

Implementation Details

• Store::Iter: Direct slice copy for Array/Vec storage, bit extraction for Bitmap storage, run expansion for Interval stores
• Container::Iter: Uses u16 buffer internally, combines with container key to produce u32 values
• Bitmap::Iter/IntoIter: Handles front/back iterators from DoubleEndedIterator and container transitions

Checklist

• Tests pass (cargo test)
• No breaking changes
• Backward compatible