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

samples/rust_generated/my_game/sample/monster_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;
 use self::flatbuffers::{EndianScalar, Follow};
 use super::*;
 pub enum MonsterOffset {}
@@ -266,8 +270,8 @@ impl<'a: 'b, 'b> MonsterBuilder<'a, 'b> {
   }
 }
 
-impl std::fmt::Debug for Monster<'_> {
-  fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+impl core::fmt::Debug for Monster<'_> {
+  fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
     let mut ds = f.debug_struct("Monster");
       ds.field("pos", &self.pos());
       ds.field("mana", &self.mana());