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/table.rs

@@ -55,14 +55,6 @@ impl<'a> Follow<'a> for Table<'a> {
     }
 }
 
-#[inline]
-pub fn get_root<'a, T: Follow<'a> + 'a>(data: &'a [u8]) -> T::Inner {
-    <ForwardsUOffset<T>>::follow(data, 0)
-}
-#[inline]
-pub fn get_size_prefixed_root<'a, T: Follow<'a> + 'a>(data: &'a [u8]) -> T::Inner {
-    <SkipSizePrefix<ForwardsUOffset<T>>>::follow(data, 0)
-}
 #[inline]
 pub fn buffer_has_identifier(data: &[u8], ident: &str, size_prefixed: bool) -> bool {
     assert_eq!(ident.len(), FILE_IDENTIFIER_LENGTH);