simplify the set/get of store, just like mobx about undux

[bcherny]

@hackerxian I did, and thanks for bringing it up! Both syntaxes are great (in fact we could easily support all 3), but there are a few small downsides, I think:

1. Having >1 way to do something makes me more uncertain how to do that thing. If I see 3 different syntaxes floating around, which is the right one? Is one a legacy syntax? Are there differences between them?
2. Using Undux in a really large codebase, a lot of people don't read docs and instead grep for usages. Assuming one form (eg. store.set(k)(v)) is the most popular in that codebase, people will use it anyway. When someone else comes along and uses a different form (eg. store.set(k, v)), it might confuse people in code review (back to (1) again).
3. Speaking of grepping for usages, if you want to see every place where your key 'userList' gets set today, you just grep for store.set('userList') and you'll get all the usages. If there are 3 forms, you'll have to grep for those too.
4. The store.set(k)(v) form encourages people to think in terms of partial application, which gives more concise, more readable code (onChange={store.set(k)}).

Admittedly, these are pretty minor downsides. I think the guiding principle is: 1 way to do things, not more.

[bcherny]

This is an interesting idea. I went ahead and implemented it as an experiment in this PR.

Three downsides of this sort of setter syntax:

1. We can't use curried setters, which are a convenient feature that works nicely with React APIs https://github.com/bcherny/undux#partial-application-all-the-way-through
2. Setting a value now looks really different than React's setState API. The similarity between .set and .setState is intentional, and I think helps people understand what they're doing.
3. It's hard to understand at a glance that you're updating an Undux store (it just looks like you're setting a property on a prop, which is bad). It's magic, which might be unnecessary.

All of these points may be reasonable compromises for the sake of simplicity. So the API would look like:

withStore('a')(store =>
  <div>a is {store.a}</div>
  <button onClick={() => store.a++}>Add 1 to a</button>
)

[hackerxian]

Do you consider this? Maybe better understand

store.set('number', 1) ;
store.set({number: 1}) ;

[zhy0216]

can we automatically generate getter/setter, but not sure if we still can get advantage of compile errors.
store.get('addTodoTitle') => store.getAddTodoTitle
store.set('addTodoTitle') => store.setAddTodoTitle

[bcherny]

@zhy0216 There's no way to do that safely, unless we introduce a codegen script.