Added ability to display function name as actionType

[Sawtaytoes]

Reason

This PR seeks to start the process of change. If the official Redux-devtools supports action types that aren't strings, then other libraries and utilities (which may already have no preference) will follow suit.

Abstract

The normal advice is that actionType should be a string. This is purely for debugging purposes. If the tools used for debugging were altered, then it would be easier to use something other than a string.

Using a function means you don't have to have a separate action variable from the action creator itself:

const someActionCreator = () => ({
  type: someActionCreator,
})

Redux doesn't care if your action type is a string, object, symbol or whatever. Even @gaearon noted it should be a Symbol for uniqueness, but only said to use strings because of the debugging potential.

Another benefit of using functions is you're only comparing memory references rather than strings. This is a performance boost, but probably not one that will manifest in most applications.

We could even take this further to include symbol.description and classInstance.constructor.name.

Downfalls

The one downfall is most Redux tooling both utilizes and expects strings. This would need to change. Even createReducer would need to take a Map of actions instead of an Object or this wouldn't work.

Alternatives

Without making a change to Redux-devtools, we could instead rely on using:

const someActionCreator = () => ({
  type: someActionCreator.name,
})

This requires knowing certain action types are functions rather than strings. In an application where you might be iteratively updating action types, this would pose a problem.