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/monster_test_serialize/my_game/example/race_generated.rs

@@ -1,7 +1,11 @@
 // automatically generated by the FlatBuffers compiler, do not modify
+extern crate alloc;
 extern crate flatbuffers;
-use std::mem;
-use std::cmp::Ordering;
+use alloc::boxed::Box;
+use alloc::string::{String, ToString};
+use alloc::vec::Vec;
+use core::mem;
+use core::cmp::Ordering;
 extern crate serde;
 use self::serde::ser::{Serialize, Serializer, SerializeStruct};
 use self::flatbuffers::{EndianScalar, Follow};
@@ -48,8 +52,8 @@ impl Race {
     }
   }
 }
-impl std::fmt::Debug for Race {
-  fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
+impl core::fmt::Debug for Race {
+  fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
     if let Some(name) = self.variant_name() {
       f.write_str(name)
     } else {