Rust: Fixed cargo clippy on non-generated code (#5485)

* Cargo clippy lints

* more lints

* more lints

* Restored a doc comment

* Comment on float eps-eq and adjusted casting

rust/flatbuffers/src/vtable_writer.rs

@@ -28,7 +28,7 @@ pub struct VTableWriter<'a> {
 impl<'a> VTableWriter<'a> {
     #[inline(always)]
     pub fn init(buf: &'a mut [u8]) -> Self {
-        VTableWriter { buf: buf }
+        VTableWriter { buf }
     }
 
     /// Writes the vtable length (in bytes) into the vtable.