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/vector.rs

@@ -14,12 +14,12 @@
  * limitations under the License.
  */
 
+use std::fmt::{Debug, Formatter, Result};
 use std::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator};
 use std::marker::PhantomData;
 use std::mem::size_of;
 use std::slice::from_raw_parts;
 use std::str::from_utf8_unchecked;
-use std::fmt::{Debug, Result, Formatter};
 
 use crate::endian_scalar::read_scalar_at;
 #[cfg(target_endian = "little")]
@@ -32,14 +32,13 @@ pub struct Vector<'a, T: 'a>(&'a [u8], usize, PhantomData<T>);
 impl<'a, T> Debug for Vector<'a, T>
 where
     T: 'a + Follow<'a>,
-    <T as Follow<'a>>::Inner : Debug
+    <T as Follow<'a>>::Inner: Debug,
 {
     fn fmt(&self, f: &mut Formatter) -> Result {
         f.debug_list().entries(self.iter()).finish()
     }
 }
 
-
 // We cannot use derive for these two impls, as it would only implement Copy
 // and Clone for `T: Copy` and `T: Clone` respectively. However `Vector<'a, T>`
 // can always be copied, no matter that `T` you have.
@@ -104,6 +103,8 @@ impl SafeSliceAccess for u8 {}
 impl SafeSliceAccess for i8 {}
 impl SafeSliceAccess for bool {}
 
+// TODO(caspern): Get rid of this. Conditional compliation is unnecessary complexity.
+// Vectors of primitives just don't work on big endian machines!!!
 #[cfg(target_endian = "little")]
 mod le_safe_slice_impls {
     impl super::SafeSliceAccess for u16 {}