Remove stray console.log
whitespace
Move re-render's isolateValue to UI; fix bugs

* Use isolateValue around `foo` in `{{> foo}}` as well as `{{#foo}}` (i.e. whether or not there's a content block)
* Make UI.With always embox its argument; this is achieved by moving embox/isolateValue from Spacebars into UI.  As a side effected, all reactive closures now use isolateValue, so we avoid re-rendering in a few more cases.
* Avoid calling data argument of `{{#with someHelper}}` twice ever instead of once ever (when helper isn't invalidated)