[compiler] Infer render helpers for additional validation (#33647)

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.

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[//]: # (BEGIN SAPLING FOOTER)
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* #33643
* #33650
* #33642
* __->__ #33647