Lots of improvements.

* Better script names and grammar fix.
* Bump Rust
* Enforce more lints.
* Improve few struct definitions by removing box.
* Many minor implementation improvements.

scripts/grade-gvn.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+
+# Exit when any command fails.
+set -e
+
+# Run lints.
+cargo fmt --all -- --check # run `cargo fmt` to auto-correct format.
+cargo clippy
+
+# Run tests.
+RUST_MIN_STACK=33554432 cargo test --release test_examples_gvn -- --nocapture