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/tests/more_defaults_test.rs

@@ -1,3 +1,8 @@
+extern crate alloc;
+
+use alloc::string::ToString;
+use alloc::vec::Vec;
+
 #[allow(dead_code, unused_imports)]
 #[path = "../../more_defaults/mod.rs"]
 mod more_defaults_generated;