Improve combineReducers type safety (TypeScript)

[drewnoakes]

This PR flows names and types of state/reducers through the combineReducers function.

Using TypeScript's Mapped Types, validate the reducer map object contains all required properties, and that the child reducers return the correct types.

For example:

interface RootType {
  users: IUser[],
  sessions: ISession[];
}

const users = (state: IUser[]) => state; // NOOP
const sessions = (state: ISession[]) => state; // NOOP

const reducer = combineReducers<RootType>({ users, sessions });

That worked before, as it does with this PR.

With this change, various errors are caught at compile time:

• Mismatched child reducer name vs. RootType property name
• Missing child reducers
• Child reducers that return different types

Current user code that doesn't specify the type on the call to combineReducers has the type inferred from the reducer map, so this change doesn't limit them, although the returned reducer's type reflects the reality of the data it will produce.

[aikoven]

Already merged into the next branch: #2182

[drewnoakes]

Thanks! Looking forward to being able to use it.