This rewrite was motivated by day8#106.

In essence re-frame provides a transducer: state, event -> state.
This transducer is able to apply algorithmic transformations on series of events
and build final state up. This allows complete decoupling of re-frame handlers,
subscriptions and middlewares logic from mechanisms how events are queued,
processed and how are their effects applied to app-db. This "bare re-frame" is
implemented in frame.cljs.

For maximal flexibility bare re-frame must be pure and have no special knowledge
of app-db, reagent/ratom and core.async. It must be possible to create multiple
independent instances of bare re-frame.

For convenience scaffold.cljs is re-implementing original re-frame functionality
of v0.4.1 using those new bare primitives. This is exposed as public api via
core.cljs.

App developer can opt-in using default implementation by requiring re-frame.core.
In this scenario re-frame provides a single app-db which is reagent's ratom and
runs single event loop where events are dispatched and processed via core.async
channel. A single re-frame instance is created and kept in
re-frame.core/app-frame atom. Plus developer gets access to the original
imperative API to manipulate them.

App developer is also free to pick/implement other means of employing bare
re-frame by not requiring re-frame.core directly and to build own scaffold
around bare re-frame primitives.

9326a41

thanks @nberger

compiler was complaining and didn't overwrite existing protocol:
WARNING: Protocol IPrintWithWriter implemented multiple times at line 35 src/re_frame/frame.cljs

but issue a warning when used

thanks @thenonameguy

as suggested by @thenonameguy

lessons learned from plastic

and give it a better name

day8#107 (comment)

dc82af6#commitcomment-12786626