templater: expand list of parsed templates to template node This patch eliminates a nested data structure other than the parsed tree. ('template', [(op, data), ..]) -> ('template', (op, data), ..) New expanded tree can be processed by common parser functions. This change will help implementing template aliases. Because a (template ..) node should have at least one child node, an empty template (template []) is mapped to (string ''). Also a trivial string (template [(string ..)]) node is unwrapped to (string ..) at parsing phase, instead of compiling phase.