Introduce the obkv heed codec

src/heed_codec/obkv_codec.rs

@@ -0,0 +1,20 @@
+use std::borrow::Cow;
+use obkv::{KvReader, KvWriter};
+
+pub struct ObkvCodec;
+
+impl<'a> heed::BytesDecode<'a> for ObkvCodec {
+    type DItem = KvReader<'a>;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        Some(KvReader::new(bytes))
+    }
+}
+
+impl heed::BytesEncode<'_> for ObkvCodec {
+    type EItem = KvWriter<Vec<u8>>;
+
+    fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
+        item.clone().into_inner().map(Cow::Owned).ok()
+    }
+}