Tweak the way scopes work

Should make it clearer what a "variable" is and allow more flexibility
in what a variable can be (previously it could only be an object, but
now it can be e.g. a reference to the template or a special shortcut)

blueprintcompiler/language/expression.py

@@ -49,9 +49,7 @@ class IdentExpr(AstNode):
         if self.is_this:
             raise CompilerBugError()
 
-        xml.start_tag("constant")
-        xml.put_text(self.tokens["ident"])
-        xml.end_tag()
+        self.parent_by_type(Scope).variables[self.tokens["ident"]].emit_xml(xml)
 
 
 class LookupOp(InfixExpr):