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/flatbuffers_benchmarks.rs

@@ -14,9 +14,8 @@
  * limitations under the License.
  */
 
+use bencher::{benchmark_group, Bencher};
 use flatbuffers;
-use bencher::Bencher;
-
 
 #[allow(dead_code, unused_imports)]
 #[path = "../../monster_test/mod.rs"]

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) {