Factor out store fx into middleware

[gordonbrander]

This PR factors out fx and logging into middleware. Store signature has changed, and includes a new parameter, middleware. Breaking change.

import {store, fxware} from 'spellcaster/spellcaster.js'

const [state, send] = store({
  state: {},
  update,
  middleware: fxware(fx)
})

This does a few things:

• The update function no longer has to think about side effects.
  • update(state: State, msg: Msg): State just returns a new state
  • next() and transactions no longer have to exist as concepts
• Fx is provided by middleware
  • A default fxware middleware is provided
  • Stores that want complex effects can plug in custom middleware
  • Middleware can be composed via middleware(...ware)
• Stores that don't need fx can ignore them. A default "no effects" middleware is provided.

[bfollington]

Nice! LGTM, not that you need my approval ;)