Add a reference section about unwinding and panics

[fitzgen]

This section should:

• Explain the current state of unwinding, ie how panic=abort is the only option.
• Describe what "aborting" means in this context, and how it turns into a JS error, and how Drop implementations are not run.
• Warn against the dangers of re-entering wasm after it has "aborted" due to inconsistent state from Drop impls not running.
• Describe how JS can handle "aborts" by creating a new instance of the wasm module.
• Briefly talk about potential future support for unwinding and link to the proposal.