formatter: remove template resources from nested items before generating JSON

templater: add public interface returning a set of resource keys The next patch depends on knownresourcekeys(), and there's no reason to keep availableresourcekeys() private.

formatter: make nested items somewhat readable in template output

templater: wrap result of '%' operation so it never looks like a thunk This fixes min/max()/json() of map result. Before, it was taken as a lazy byte string and stringified by evalfuncarg().