Introduce setState(fn) variant

[LPGhatguy]

In order to implement asynchronous rendering (#18), we need to make sure there's an API in place that mirror's React's setState variant that accepts a function.

Dependent updates work just fine right now, which means this code that works today would not work with asynchronous rendering:

setState(self.state.value + 1)

Instead, we should introduce a new API and encourage users to use it today:

setState(function(props, state)
    return {
        value = state.value + 1
    }
end)

Once work on async rendering begins, we can start catching cases where users call setState multiple times in a row synchronously to try to weed out dependent state mutations.