Refactor facet-related codecs

milli/src/heed_codec/facet/str_ref.rs

@@ -0,0 +1,20 @@
+use std::borrow::Cow;
+
+use heed::{BytesDecode, BytesEncode};
+
+pub struct StrRefCodec;
+impl<'a> BytesEncode<'a> for StrRefCodec {
+    type EItem = &'a str;
+
+    fn bytes_encode(item: &'a &'a str) -> Option<Cow<'a, [u8]>> {
+        Some(Cow::Borrowed(item.as_bytes()))
+    }
+}
+impl<'a> BytesDecode<'a> for StrRefCodec {
+    type DItem = &'a str;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Option<Self::DItem> {
+        let s = std::str::from_utf8(bytes).unwrap();
+        Some(s)
+    }
+}