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/builder.rs

@@ -435,7 +435,7 @@ impl<'fbb> FlatBufferBuilder<'fbb> {
         // Write the vtable offset, which is the start of any Table.
         // We fill its value later.
         let object_revloc_to_vtable: WIPOffset<VTableWIPOffset> =
-            WIPOffset::new(self.push::<UOffsetT>(0xF0F0_F0F0 as UOffsetT).value());
+            WIPOffset::new(self.push::<UOffsetT>(0xF0F0_F0F0).value());
 
         // Layout of the data this function will create when a new vtable is
         // needed.