Rust codegen improvements and lint fixes (#6046)

* Improve rust codegen

* one more unneeded lifetime

* Added a derive default

Co-authored-by: Casper Neo <cneo@google.com>

tests/optional_scalars_generated.rs

@@ -28,9 +28,7 @@ impl<'a> flatbuffers::Follow<'a> for ScalarStuff<'a> {
     type Inner = ScalarStuff<'a>;
     #[inline]
     fn follow(buf: &'a [u8], loc: usize) -> Self::Inner {
-        Self {
-            _tab: flatbuffers::Table { buf: buf, loc: loc },
-        }
+        Self { _tab: flatbuffers::Table { buf, loc } }
     }
 }