Rust Flatbuffers Verifier (#6269)

* Updated comments and fixed a fundemental type error.

* bump rust flatbuffers semver

* Initial commit with verifier, need to clean up

* Verifier tested. Needs clean up and refactoring.

* Display for InvalidFlatbuffer and better errors for strings

* SimpleToVerify, some refactoring

* Combined VerifierType TableAccessorFuncBody into FollowType

* scrub todos

* Update Rust get_root functions.

There are 6 variants, with verifier options, default verifier options
and no verification "fast".

* Rename root fns

* inline

* Update to use thiserror

* fix for bad compiler

* improve error formatting

* Replace multiply with saturating_multiply

* saturating adds too

* Add docs disclaiming experimental verification system

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

tests/rust_usage_test/bin/flatbuffers_alloc_check.rs

@@ -131,7 +131,7 @@ fn main() {
 
         // do many reads, forcing them to execute by using assert_eq:
         {
-            let m = my_game::example::get_root_as_monster(buf);
+            let m = unsafe { my_game::example::root_as_monster_unchecked(buf) };
             assert_eq!(80, m.hp());
             assert_eq!(150, m.mana());
             assert_eq!("MyMonster", m.name());

tests/rust_usage_test/bin/monster_example.rs

@@ -20,7 +20,7 @@ fn main() {
     let mut buf = Vec::new();
     f.read_to_end(&mut buf).expect("file reading failed");
 
-    let monster = my_game::example::get_root_as_monster(&buf[..]);
+    let monster = my_game::example::root_as_monster(&buf[..]).unwrap();
     println!("{}", monster.hp()); // `80`
     println!("{}", monster.mana()); // default value of `150`
     println!("{:?}", monster.name()); // Some("MyMonster")