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>

samples/sample_binary.rs

@@ -20,8 +20,7 @@ extern crate flatbuffers;
 // import the generated code
 #[path = "./monster_generated.rs"]
 mod monster_generated;
-pub use monster_generated::my_game::sample::{get_root_as_monster,
-                                             Color, Equipment,
+pub use monster_generated::my_game::sample::{Color, Equipment,
                                              Monster, MonsterArgs,
                                              Vec3,
                                              Weapon, WeaponArgs};
@@ -98,7 +97,7 @@ fn main() {
   let buf = builder.finished_data(); // Of type `&[u8]`
 
   // Get access to the root:
-  let monster = get_root_as_monster(buf);
+  let monster = flatbuffers::root::<Monster>(buf).unwrap();
 
   // Get and test some scalar types from the FlatBuffer.
   let hp = monster.hp();