Reduce re-renders by auto detecting state dependencies

[shuding]

By moving data, error, isValidating away from the state (useReducer) to a ref (stateRef), the hook won't trigger another re-render anymore when one of these values updates.

Instead, we add getters to these values (implemented with defineProperties). Once a value is read, it will be marked as a dependency immediately. Thus, we can trigger a re-render if any dependency changes in the future.

With this change, useSWR will never cause unnecessary re-renders:

const { data } = useSWR(...)
// will not re-render if `isValidating` or `error` changes

No changes needed from the user side (and Suspense mode is not affected).

Fixes #184. Related #144, #114.

Published at swr@0.1.13-beta.0 swr@0.1.15-beta.0.

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This PR has been running on our production for a couple of weeks, and it works very well 🎉

[shuding]

Added more code optimization (memorize returned state and dispatch function), also fixed a bug causing SWRPages not updating in 5795f33.

Published a new version based on the current master to swr@0.1.15-beta.0.

[shuding]

Note: in the future we are going to simplify the useSWRPages hook (which is undocumented currently).

[iamstarkov]

Seems like this PR breaks ssr when initialData is used. see #230