The type parameter 'C' will be removed from the Template trait, making it
represent a printable type or compiled template.
TemplateRenderer now holds Box<dyn _> template because it's unlikely that the
inner template type can be statically determined.
In short, this enables compilation of template of e.g. Vec<Commit> type by
using CommitTemplateLanguage.
A Template<C> was originally compiled for a specific type C, and invoked as
"Fn(&C) -> _". It was simple and intuitive, but we had to define the context
type C statically. Things got even worse by extensions support because we had
to provide object-safe hook point for each context type C.
This patch basically removes the Context type from compiled templates. The
"self" variable is injected through RefCell<Option<C>> placeholder. A compiled
template knows its "self" type, but the type can be decided per instance, not
per TemplateLanguage type. A drawback is that the "self" variable will have to
be cloned one more time.
The Template<C> abstraction no longer makes sense, and will be split to inner
Template<()> implementations (which usually represent printable types) and the
outer Template<C>.
