Implement bitset

CRoaring introduced a `bitset` type, and the abilty to convert a roaring
bitmap into a dense bitset. This adds wrappers around the new bitset
functions

croaring/src/bitmap/imp.rs

@@ -1,3 +1,4 @@
+use crate::Bitset;
 use ffi::roaring_bitmap_t;
 use std::convert::TryInto;
 use std::ffi::c_char;
@@ -1486,6 +1487,29 @@ impl Bitmap {
 
         statistics
     }
+
+    /// Store the bitmap to a bitset
+    ///
+    /// This can be useful for those who need the performance and simplicity of a standard bitset.
+    ///
+    /// # Errors
+    ///
+    /// This function will return None on allocation failure
+    ///
+    /// # Examples
+    /// ```
+    /// use croaring::Bitmap;
+    /// let bitmap = Bitmap::from_range(0..100);
+    /// let bitset = bitmap.to_bitset().unwrap();
+    /// assert_eq!(bitset.count(), 100);
+    /// ```
+    #[inline]
+    #[doc(alias = "roaring_bitmap_to_bitset")]
+    pub fn to_bitset(&self) -> Option<Bitset> {
+        let mut bitset = Bitset::new();
+        let success = unsafe { ffi::roaring_bitmap_to_bitset(&self.bitmap, bitset.as_raw_mut()) };
+        success.then_some(bitset)
+    }
 }
 
 fn range_to_inclusive<R: RangeBounds<u32>>(range: R) -> (u32, u32) {

croaring/src/bitmap/iter.rs

@@ -3,6 +3,7 @@ use std::marker::PhantomData;
 
 use super::Bitmap;
 
+/// Iterator over the values of a bitmap
 #[derive(Clone)]
 pub struct BitmapIterator<'a> {
     iterator: ffi::roaring_uint32_iterator_s,

croaring/src/bitmap/lazy.rs

@@ -1,5 +1,6 @@
 use crate::Bitmap;
 
+/// A handle to lazily perform multiple bitwise operations on a bitmap
 pub struct LazyBitmap<'a> {
     bitmap: &'a mut Bitmap,
 }

croaring/src/bitmap/mod.rs

@@ -55,6 +55,7 @@
 
 use std::marker::PhantomData;
 
+/// A compressed bitmap
 // Must be repr(transparent) and match BitmapView, to allow safe transmute between
 // &BitmapView and &Bitmap
 #[repr(transparent)]
@@ -78,6 +79,7 @@ pub struct BitmapView<'a> {
 unsafe impl<'a> Sync for BitmapView<'a> {}
 unsafe impl<'a> Send for BitmapView<'a> {}
 
+/// Detailed statistics on the composition of a bitmap
 pub type Statistics = ffi::roaring_statistics_s;
 
 mod imp;