Fix Rust UB problems (#6393)

* Fix miri problems by assuming alignment is 1 in rust

* Removed is_aligned fn from rust verifier.

* Add back is_aligned, but make it w.r.t. buffer[0]

* touch unused variable

* touch unused variable

* +nightly

* Move Rust miri testing into its own docker

* fix bash

* missing one endian conversion

* fix endianness2

* format stuff

Co-authored-by: Casper Neo <cneo@google.com>

tests/rust_usage_test/tests/flexbuffers_tests/other_api.rs

@@ -13,9 +13,11 @@
 // limitations under the License.
 
 use flexbuffers::*;
+#[cfg(not(miri))]  // slow.
 use quickcheck::QuickCheck;
 
 #[test]
+#[cfg(not(miri))]  // slow.
 fn qc_reader_no_crash() {
     fn no_crash(xs: Vec<u8>) -> bool {
         let r = Reader::get_root(&xs);