Changed return_ref syntax to returns(as_ref) and returns(cloned) (#772)

* Changed `return_ref` syntax to `returns(as_ref)` and `returns(cloned)`

* Implement

* renamed module for return_mode

* Rename macro, fix docs, add tests, validate return modes

* Cargo fmt

---------

Co-authored-by: Micha Reiser <micha@reiser.io>

examples/lazy-input/main.rs

@@ -67,7 +67,7 @@ fn main() -> Result<()> {
 #[salsa::input]
 struct File {
     path: PathBuf,
-    #[return_ref]
+    #[returns(ref)]
     contents: String,
 }
 
@@ -158,7 +158,7 @@ impl Diagnostic {
 #[salsa::tracked]
 struct ParsedFile<'db> {
     value: u32,
-    #[return_ref]
+    #[returns(ref)]
     links: Vec<ParsedFile<'db>>,
 }