templater: convert resources to a table of callables for future extension I'm going to add a full templating support to the annotate command. As the annotate is a filectx-oriented command, we'll need a way to look up a ctx from a bounded fctx only when necessary. This is the minimal change to support that. I'm thinking of defining a proper interface to look up template resources to fix other issues, but that isn't ready yet. (Changes from V1: just updated tests and patch descriptions.)