This commit passes the template format to the digestor in order to come
up with a key. Before this commit, the digestor would depend on the
side effect of the template renderer setting the rendered_format on the
lookup context. I would like to remove that mutation, so I've changed
this to pass the template format in to the digestor.
I've introduced a new instance variable that will be alive during a
template render. When the template is being rendered, it pushes the
current template on to a stack, setting `@current_template` to the
template currently being rendered. When the cache helper asks the
digestor for a key, it uses the format of the template currently on the
stack.