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:
     {

tests/rust_usage_test/bin/flexbuffers_alloc_check.rs

@@ -103,6 +103,7 @@ fn validate_monster(flexbuffer: &[u8]) {
 
 // This is in a separate binary than tests because taking over the global allocator is not
 // hermetic and not thread safe.
+#[cfg(not(miri))]  // slow.
 fn main() {
     let start_up = current_allocs();
 
@@ -133,6 +134,7 @@ fn main() {
 }
 
 #[test]
+#[cfg(not(miri))]  // slow.
 fn no_extra_allocations() {
     main()
 }