Add an XXX for failing test
Fix broken newargs semantics

All tests pass (except one which is a todo on shark).

If `{{> foo bar}}` is to desugar into `{{#with bar}}{{> foo}}{{/with}}`, there needs to be an explicit block in the generated code in which `self.lookup("foo")` binds the correct data context (`bar`).  So we now generate a UI.With rather than trying to call it from Spacebars.include.  This simplifies Spacebars.include, which is nice.

Define `lookupTemplate` because `self.lookup(.., {template: true})` pretty-prints bigly.