formatter: provide hint of referenced field names I don't like the function name, but I call it datahint() for consistency with another function I'll add later, fm.contexthint().

templater: add function to look up symbols used in template Formatter can use this information to enable slow paths such as loading ctx object only when necessary.

templater: parse template string to tree by templater class The parsed tree could be cached, but it isn't for now. We can add a cache later if that matters.