Move StrRefCodec and ByteSliceRefCodec to their own files

milli/src/heed_codec/byte_slice_ref.rs

@@ -0,0 +1,23 @@
+use std::borrow::Cow;
+
+use heed::{BytesDecode, BytesEncode};
+
+/// A codec for values of type `&[u8]`. Unlike `ByteSlice`, its `EItem` and `DItem` associated
+/// types are equivalent (= `&'a [u8]`) and these values can reside within another structure.
+pub struct ByteSliceRefCodec;
+
+impl<'a> BytesEncode<'a> for ByteSliceRefCodec {
+    type EItem = &'a [u8];
+
+    fn bytes_encode(item: &'a Self::EItem) -> Option<Cow<'a, [u8]>> {
+        Some(Cow::Borrowed(item))
+    }
+}
+
+impl<'a> BytesDecode<'a> for ByteSliceRefCodec {
+    type DItem = &'a [u8];
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        Some(bytes)
+    }
+}