Put the documents MTBL back into LMDB

We makes sure to write the documents into a file before
memory mapping it and putting it into LMDB, this way we avoid
moving it to RAM

src/heed_codec/mod.rs

@@ -1,5 +1,7 @@
+mod mtbl_codec;
 mod roaring_bitmap_codec;
 mod str_beu32_codec;
 
+pub use self::mtbl_codec::MtblCodec;
 pub use self::roaring_bitmap_codec::RoaringBitmapCodec;
 pub use self::str_beu32_codec::StrBEU32Codec;

src/heed_codec/mtbl_codec.rs

@@ -0,0 +1,20 @@
+use std::borrow::Cow;
+use oxidized_mtbl::Reader;
+
+pub struct MtblCodec;
+
+impl<'a> heed::BytesDecode<'a> for MtblCodec {
+    type DItem = Reader<&'a [u8]>;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        Reader::new(bytes).ok()
+    }
+}
+
+impl heed::BytesEncode<'_> for MtblCodec {
+    type EItem = [u8];
+
+    fn bytes_encode(item: &Self::EItem) -> Option<Cow<[u8]>> {
+        Some(Cow::Borrowed(item))
+    }
+}