note: This implements the idea discussed in reactwg/react#389 (comment)

Currently we create `Impure` effects for impure functions like `Date.now()` or `Math.random()`, and then throw if the effect is reachable during render. However, impurity is a property of the resulting value: if the value isn't accessed during render then it's okay: maybe you're console-logging the time while debugging (fine), or storing the impure value into a ref and only accessing it in an effect or event handler (totally ok).

This PR updates to validate that impure values are not transitively consumed during render, building on the `Render` effect. The validation currently uses an algorithm very similar to that of InferReactivePlaces - building a set of known impure values, starting with `Impure` effects as the sources and then flowing outward via data flow and mutations. If a transitively impure value reaches a `Render` effect, it's an error. We record both the source of the impure value and where it gets rendered to make it easier to understand and fix errors:

```
Error: Cannot access impure value during render

Calling an impure function can produce unstable results that update unpredictably when the component happens to re-render. (https://react.dev/reference/rules/components-and-hooks-must-be-pure#components-and-hooks-must-be-idempotent).

error.invalid-impure-functions-in-render-via-render-helper.ts:10:25
   8 |     const array = makeArray(now);
   9 |     const hasDate = identity(array);
> 10 |     return <Bar hasDate={hasDate} />;
     |                          ^^^^^^^ Cannot access impure value during render
  11 |   };
  12 |   return <Foo renderItem={renderItem} />;
  13 | }

error.invalid-impure-functions-in-render-via-render-helper.ts:6:14
  4 |
  5 | function Component() {
> 6 |   const now = Date.now();
    |               ^^^^^^^^^^ `Date.now` is an impure function.
  7 |   const renderItem = () => {
  8 |     const array = makeArray(now);
  9 |     const hasDate = identity(array);
```

Impure values are allowed to flow into refs, meaning that we now allow `useRef(Date.now())` or `useRef(localFunctionThatReturnsMathDotRandom())` which would have errored previously. The next PR reuses this improved impurity tracking to validate ref access in render as well.