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/bin/flatbuffers_alloc_check.rs

@@ -98,6 +98,7 @@ fn create_serialized_example_with_generated_code(builder: &mut flatbuffers::Flat
     my_game::example::finish_monster_buffer(builder, mon);
 }
 
+#[cfg(not(miri))]  // slow.
 fn main() {
     // test the allocation tracking:
     {