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>

rust/flatbuffers/src/follow.rs

@@ -41,7 +41,7 @@ pub fn lifted_follow<'a, T: Follow<'a>>(buf: &'a [u8], loc: usize) -> T::Inner {
 
 /// FollowStart wraps a Follow impl in a struct type. This can make certain
 /// programming patterns more ergonomic.
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct FollowStart<T>(PhantomData<T>);
 impl<'a, T: Follow<'a> + 'a> FollowStart<T> {
     #[inline]