Simplify applyMiddleware

[jednano]

Breaking up #3566 into smaller pieces.

This PR simplifies the types of applyMiddleware by replacing all the overloads with a single generic function signature that infers the union of any/all Dispatch types provided into a single StoreEnhancer. This implementation has the added benefit that you can provide as many middlewares as you want w/o maxing out at just 5.

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[timdorr]

Looks good and much easier to read through this way. Thanks, Jed!

[timdorr]

Why does this need to be generic? In what case would our middleware not be a Middleware? Either we don't need this, or we need to add a test for this generic.

[jednano]

If it's not generic, I can't infer off of it; rather, I do get errors if I try to do typeof middlewares extends Middleware<any, any, infer D>.

Type '(createStore: StoreEnhancerStoreCreator<any, never>) => <S = any, A extends Action<any> = AnyAction>(reducer: Reducer<S, A>, ...args: any[]) => any' is not assignable to type 'StoreEnhancer<never, S>'.
  Type '<S = any, A extends Action<any> = AnyAction>(reducer: Reducer<S, A>, ...args: any[]) => any' is not assignable to type 'StoreEnhancerStoreCreator<never, S>'.
    Type 'any' is not assignable to type 'never'.ts(2322)

It will always be a Middleware, which is why I said it must extend Middleware, but there's no reason someone can't create an interface which extends Middleware and use that too.

[jednano]

Also, we already have tests against this generic in the existing applyMiddleware.spec.ts which pass just fine with npm run test:cov for some reason; though, I am seeing red underlines in my editor (even before my changes here). I've fixed those red-line errors in other pieces of my work, but once I started breaking things apart, it's a bit harder to compose these things into smaller chunks w/o some overlap.

[jednano]

Let me give a stab at some tests.

[jednano]

Nope, this doesn't work the way I thought. The tests prove that. Side note, it doesn't work this way before my changes either. Should I submit a PR with breaking tests to show you what I mean? Or is my assumption incorrect?

  it('combines Dispatch types from all middlewares', () => {
    const foo: Middleware<any, any, Dispatch<{ type: 'foo' }>> = ({
      dispatch
    }) => _next => _action => dispatch({ type: 'foo' })
    const bar: Middleware<any, any, Dispatch<{ type: 'bar' }>> = ({
      dispatch
    }) => _next => _action => dispatch({ type: 'bar' })

    const store = createStore(x => x, applyMiddleware(foo, bar))

    store.dispatch({ type: 'foo' }) // OK
    store.dispatch({ type: 'bar' })  // OK
    store.dispatch({ type: 'baz' }) // not OK
  })

The part TS doesn't like is applyMiddleware(foo, bar), because bar isn't of the same type as foo. Need to rethink this a bit, I think.