Fix flow types for createRPCReducer interface

[ganemone]

This fixes issues with the createRPCReducer interface when working with flow types by using generics rather than the existential operator * whenever possible. This allows you to create a flow typed rpc reducer using the Reducer type helper from redux. For example:

import type {Reducer} from 'redux';
import {createRPCReducer} from 'fusion-rpc-redux';

const StateType = {|
  a: string,
  b: boolean
|};

const ActionType = {|
  type: string,
  payload: {
    a: string,
    b : boolean
  }
|}

const reducer: Reducer<StateType, ActionType> = createRPCReducer('rpcId', {
  // ...
});

[codecov]

Codecov Report

Merging #108 into master will not change coverage.
The diff coverage is 91.66%.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   96.66%   96.66%           
=======================================
  Files           1        1           
  Lines          60       60           
  Branches       16       12    -4     
=======================================
  Hits           58       58           
  Misses          1        1           
  Partials        1        1

Impacted Files	Coverage Δ
src/index.js	96.66% <91.66%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update cce3127...c386d81. Read the comment docs.

[KevinGrandon]

Nice job on the flow fixture tests!

[KevinGrandon]

Question - Why is this needed if we're running flow in the testing suite?

[ganemone]

The fixtures are run separately, and some of them are intended to fail. We want that to show up in tests but not when running flow on the project