When a component suspends, under some conditions, we can wait for the
data to resolve and replay the component without unwinding the stack or
showing a fallback in the interim. When we do this, we reuse the
promises that were unwrapped during the previous attempts, so that if
they aren't memoized, the result can still be used.
We should do the same for all hooks. That way, if you _do_ memoize an
async function call with useMemo, it won't be called again during the
replay. This effectively gives you a local version of the functionality
provided by `cache`, using the normal memoization patterns that have
long existed in React.
Andrew Clark
8b44359178