Emily Rust Lambda Boilerplate (#101)

* Add NotImplementedError to Smithy API definition.

* Create rust lambda boilerplace.

* Fix CI by forcing autogenerated sources to be made before cargo test.

* Remove incorrect comments.

* remove unnecessary unused_extern_crates flag.

* Address style changes and link TODO ticket

* Move main to be the first function in the lambda.

.github/workflows/rust-tests.yaml

@@ -29,4 +29,8 @@ jobs:
 
       - uses: Swatinem/rust-cache@v2
 
+      # Build Emily first so that the autogenerated `lib.rs` file is
+      # present when building the rest of the crates in the workspace.
+      - run: cargo build --package emily
+
       - run: cargo test