Asynchronous functions can mutably access the same memory simultaneously

[Xaeroxe]

Example project: napi-mut-twice.zip

This is in a similar vein to #1338. To run the example

yarn run build

This runs correctly, nothing is wrong.

yarn run x

This mutably borrows data twice, something is wrong.

yarn run x2

Expected behavior: I'm not sure. More on this later.

Actual behavior: In yarn run x we can see the code behaving in a normal and intended way. In x2 however, the tokio runtime has two mutable borrows to the same data, which results in undefined and unsound behavior. This is not allowed per Chapter 4 section 2 of the Rust Book.

So what should we do about this? I have some ideas with varying trade offs.

1. Forbid &mut _ in async #[napi] methods.

   Pros: JavaScript never has to think about Rust borrow rules and restrictions
   Cons: Rust users can't use &mut _ with async functions anymore.

2. Use RefCell or something like it to dynamically check borrow rules.

   Pros: We can keep writing async functions with &mut _ arguments.
   Cons: JavaScript users are now exposed to the restrictions of Rust and all that entails in a likely surprising way.

My personal preference is for option 1.