Make Views Great Again

View functions with a subscription are not pure.  They are obtaining an "input" from a non argument source.

From a practical point view, I haven't been motivated to solve this problem - non-local reasoning is not a problem, and neither is testing because subscribe is easy to stub out,  so it feels like you are dealing with a pure function - HOWEVER from a theoretical point of view, this is certainly a "pebble in the shoe".  In due course, I'd like it fixed. 

Here's a quick sketch of approximated how (not all issues resolved):

1. create a new `reg-view` function thru which all views are associated with an `id`.  Make this registration much like `reg-sub` in the way that you can nominate N signal inputs via a signals function.
2.  change Reagent so that the first element of vector can be an `id` previously registered by `reg-view`. So, instead of `:div`, you could use `:something/panelX`

Would be used like this (notice how similar this is to reg-sub):

```
(re-frame.core/reg-view           ;;  <--- new 
   :a-view-id                              ;; an id for this view

   ;; input signals function 
   ;; what it returns will become the first param passed to the computation function
   (fn  [item-id  another]         ;; given props 
        (subscribe [:item item-id]))

   ;; computation function which returns hiccup
   (fn [item item-id another]     ;; given props BUT with values from subscription prepended
      [:div (:name item)]))
```
The computation becomes pure.  The "subscribed to' values are put in the first argument (same pattern as is currently used with `reg-sub`). 

Later, when you wanted to use this view:
`[:a-view-id  12  "hello"]`
Reagent would look up the view id `a-view-id` and use the associated 

Open Questions: 

- how to do Form-2 views or Form-3?  This could be a stumbling block

Implementation: