Potentially migrate to setState instead of replaceState

[ethul]

Currently, replaceState is used in the writeState function.

purescript-react/src/React.js

Lines 35 to 37 in cbe6f58

	ctx.replaceState({
	state: state
	});

However, from the docs, replaceState may be removed in the future.
https://facebook.github.io/react/docs/component-api.html#replacestate

Since the state is wrapped in {state: state} I believe we'd get the same behaviour even though setState does a shallow merge instead of a complete state replacement.

I hope to have a chance to prepare a PR for this. But I wanted to note it down.

[paf31]

👍 Sounds good.