Add section about debugging memory leaks

[fitzgen]

Recently we gained two new helpful tools for debugging memory leaks:

• Utility for counting the number of live JsValues in the wasm-bindgen heap wasm-bindgen#1521
• https://github.com/rustwasm/wasm-tracing-allocator

We should add a reference guide for boosting their visibility and explaining how to use them a little bit. Maybe even have an example code that intentionally leaks, use the tools to diagnose the leak, and then show the fix.