Make flatc generate Rust files not requiring std (#7273)

* Set an explicit 2018 edition for Rust tests

* Replace all `std` usage with `core` and `alloc` in Rust code generator

* Update the generated files

* Make Rust tests actually use no_std when the corresponding feature is enabled

tests/rust_usage_test/benches/flexbuffers_benchmarks.rs

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use bencher::Bencher;
+use bencher::{benchmark_group, benchmark_main, Bencher};
 use flexbuffers::*;
 
 fn push_vec_u64_to_map(b: &mut Bencher) {