Tower of hacks to get shark deps right

The implementation is getting really, really ugly.  Just want to get to correctness.

We now stop autoruns not just when started from `materialize` but also `toHTML`, `toText`, and `evaluate`, which is important for HTML attributes.  Reactive regions inside attributes aren't individually autorun; the design is that their dependencies will cause the entire attribute updater to be invalidated and re-run.  In other words, HTML.toText doesn't have internal re-runs but it does register dependencies on the current computation.  In fact, however, the presence of emboxedValues means that there *are* autoruns that need to be stopped, even in toText and evaluate.

The hack where we define a `.materialized()` callback for the benefit of UI.With falls short, because toText et al. don't call it.  The current workaround is yet worse hacks in htmljs (stopWithLater).

Finally, the fact that UI.If, UI.Unless, and Spacebars.include return reactive closures that close over emboxedValues is a problem when those same closures are reused in recalculating attributes.  The intent was that recalculating attributes should tear down any boxes internal to the attribute calculation and start fresh, but when we reuse closures that close over boxes, we are reusing boxes, and if those boxes have been stopped we lose correctness.  The ugly hack to get this to work for now is to have the boxes in If, Unless and include not be per reactive closure but per currentComputation, i.e. per autorun.  Since toText et al. don't normally autorun reactive closures, we add an autorun so that they get their own Computation objects.  This hack is supported by UI.namedEmboxValue and callReactiveFunction.

The good news is that the right answer is buried in here somewhere.