Implement Debug trait for Rust flatbuffers. (#6207)

* Refactor idl_gen_rust to a ForAllX continuation pattern.

* Removed unneeded SetValue and updated sample rust gencode

* Make Rust flatbuffers print right

* Generated code and removed unnecessary trait constraint

* bumped rust version. Release required

* removed an unwrap in Rust Debug-print unions

* Tested formatting flatbuffers in rust.

* Set float precision in flaky debug-print test

* impl Debug for structs too

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

samples/sample_binary.rs

@@ -152,6 +152,7 @@ fn main() {
   //assert_eq!(monster.path().unwrap()[1].x(), 4.0);
 
   println!("The FlatBuffer was successfully created and accessed!");
+  dbg!(monster);
 }
 
 #[cfg(test)]