Move crates under a sub folder to clean up the code

crates/milli/src/heed_codec/str_ref.rs

@@ -0,0 +1,21 @@
+use std::borrow::Cow;
+
+use heed::{BoxedError, BytesDecode, BytesEncode};
+
+/// A codec for values of type `&str`. Unlike `Str`, its `EItem` and `DItem` associated
+/// types are equivalent (= `&'a str`) and these values can reside within another structure.
+pub struct StrRefCodec;
+impl<'a> BytesEncode<'a> for StrRefCodec {
+    type EItem = &'a str;
+
+    fn bytes_encode(item: &'a &'a str) -> Result<Cow<'a, [u8]>, BoxedError> {
+        Ok(Cow::Borrowed(item.as_bytes()))
+    }
+}
+impl<'a> BytesDecode<'a> for StrRefCodec {
+    type DItem = &'a str;
+
+    fn bytes_decode(bytes: &'a [u8]) -> Result<Self::DItem, BoxedError> {
+        std::str::from_utf8(bytes).map_err(Into::into)
+    }
+}