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/Cargo.toml

@@ -4,6 +4,7 @@ version = "0.1.0"
 authors = ["Robert Winslow <hello@rwinslow.com>",
            "Casper Neo <cneo@google.com>",
            "FlatBuffers Maintainers"]
+edition = "2018"
 
 [dependencies]
 flatbuffers = { path = "../../rust/flatbuffers", default-features = false }