UI.render/toHTML, TemplateWith, Blaze.toText

Move Spacebars.TemplateWith to Blaze._TemplateWith.  It is the official
way to give a template data, after all, but it’s a little too weird to
make public.  Use it in Blaze.renderWithData, even though it doesn’t
seem like it would make a difference.

Unify UI.render/renderWithData with Blaze.render, and add
toHTMLWithData.  Publicly, they take a Template or a View.
The View must be unrendered, of course.  If given a Template,
constructView will be called.  Privately, they take any HTMLJS
extended to be allowed to contain Templates and Views, except
null or undefined, or a function returning extended HTMLJS.
The “data” argument may be a function or a not, as in UI.With
(and unlike the old public UI.renderWithData).

Make Blaze.toText private (Blaze._toText).