implement combineReducers

[AmaranthineCodices]

This fixes #6. It adds a combineReducers function that works pretty much as expected:

combineReducers({
    a = function(state, action)
        -- state here is rootState.a
    end,
    b = function(state, action)
        -- state here is rootState.b
    end,
})

[coveralls]

Coverage increased (+0.08%) to 98.947% when pulling 745de3a on AmaranthineCodices:combineReducers into 676e5d3 on Roblox:master.

[LPGhatguy]

Awesome PR! Minor test request.

[LPGhatguy]

For pedantry's sake, can we actually return a new state value for each of these to test that values are assigned to the correct key? Something simple, like making a return state + 1 and b returning state + 2 would do.

[AmaranthineCodices]

Should be good now; I added a separate test for this.

[LPGhatguy]

Thanks, as always!