Create an encapsulated "Dust template" class

Dust templates currently compile to a set of functions that are invoked via `body_0` being registered by `dust.register`.

In #513 we suggest that these body functions could be members of a "Body" class so that we don't have to set magic flags on them to distinguish them from regular functions. In addition, `.render()` methods could be part of the class instead of invoking the function directly, etc.

Read issue #513 first, then trace how Dust templates compile body_0 functions and how dust.register invokes them. The work is done when template bodies are encapsulated by a Body class and rendering uses class methods rather than magic flags or direct function invocation.

Written by the indexing model from the issue text.