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>

rust/flatbuffers/src/follow.rs

@@ -27,16 +27,9 @@ use std::marker::PhantomData;
 /// Writing a new Follow implementation primarily involves deciding whether
 /// you want to return data (of the type Self::Inner) or do you want to
 /// continue traversing the FlatBuffer.
-pub trait Follow<'a> {
+pub trait Follow<'buf> {
     type Inner;
-    fn follow(buf: &'a [u8], loc: usize) -> Self::Inner;
-}
-
-/// Execute a follow as a top-level function.
-#[allow(dead_code)]
-#[inline]
-pub fn lifted_follow<'a, T: Follow<'a>>(buf: &'a [u8], loc: usize) -> T::Inner {
-    T::follow(buf, loc)
+    fn follow(buf: &'buf [u8], loc: usize) -> Self::Inner;
 }
 
 /// FollowStart wraps a Follow impl in a struct type. This can make certain