[compiler] Infer render helpers for additional validation

We currently assume that any functions passes as props may be event handlers or effect functions, and thus don't check for side effects such as mutating globals. However, if a prop is a function that returns JSX that is a sure sign that it's actually a render helper and not an event handler or effect function. So we now emit a `Render` effect for any prop that is a JSX-returning function, triggering all of our render validation.

This required a small fix to InferTypes: we weren't correctly populating the `return` type of function types during unification. I also improved the printing of types so we can see the inferred return types.