Thanks!

So `items.[Iterable.iterator]()` is invoking a symbol-keyed method.

It's declared like:

    export interface Iterable<T> {
      static symbol iterator;

      [iterator](): Iterator<T>;
    }

    export MyArray<T> implements Iterable<T> {
      [Iterable.iterator]() { ... }
    }

I do want to add pure functions, especially for compile time constants. I want to add a macro system that can either run pure functions (on the AST or IR, not sure yet) at compile time, or run arbitrary code sandboxed in a Wasm module.