templater: add class representing a nested mappings The mappinggenerator class is necessary to fix hgweb bugs without BC. The mappinglist is for nested formatter items. They are similar, so factored out the base class. The mappinglist could be implemented by using the mappinggenerator, but we'll probably need a direct access to the raw list, so they are implemented as separate classes. Note that tovalue() isn't conforming to the spec yet in that it may return a list of dicts containing unprintable resources. This problem will be fixed later. Tests will be added by subsequent patches.