[core]: Program-bound state variables.

We have state maps and state sets. What I'm suggesting is an even simpler primitive which is a state variable.

```ts
const [get, set] = useStateVariable(key, initialValue);

get(program);

set(program, value);
```

I'm currently using a pattern pretty frequently that looks like:

```ts
const KEY = Symbol.for(...);

interface Store {
[KEY]?: T;
}

function get(program): T | undefined {
return (program as unknown as Store)[KEY];
}

function set(program, value) {
(program as unknown as Store)[KEY] = value;
}
```

This lets me ephemerally associate arbitrary state with the Program in a way that simple state sets and maps don't (because they require that the state be associated with a Type).