Introduce the roaring bitmap heed codec

src/heed_codec/mod.rs

@@ -0,0 +1,3 @@
+mod roaring_bitmap;
+
+pub use self::roaring_bitmap::RoaringBitmapCodec;

src/heed_codec/roaring_bitmap.rs

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+use std::borrow::Cow;
+use roaring::RoaringBitmap;
+
+pub struct RoaringBitmapCodec;
+
+impl heed::BytesDecode<'_> for RoaringBitmapCodec {
+    type DItem = RoaringBitmap;
+
+    fn bytes_decode(bytes: &[u8]) -> Option<Self::DItem> {
+        RoaringBitmap::deserialize_from(bytes).ok()
+    }
+}
+
+impl heed::BytesEncode<'_> for RoaringBitmapCodec {
+    type EItem = RoaringBitmap;
+
+    fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
+        let mut bytes = Vec::new();
+        item.serialize_into(&mut bytes).ok()?;
+        Some(Cow::Owned(bytes))
+    }
+}