Previously it's possible to have multiple copies of the "same" Template.
For example, if index.html.erb is found both the :en and :fr locale, it
will return a different Template object for each. The same can happen
with formats, variants, and handlers.
This commit de-duplicates templates, there will now only be one template
per file/virtual_path/locals tuple.
We need to consider virtual_path because both `render "index"`, and
`render "index.html"` can both find the same file but will have
different virtual_paths. IMO this is rare and should be
deprecated/removed, but it exists now so we need to consider it in order
to cache correctly.
This commit introduces a new UnboundTemplate class, which represents a
template with unknown locals. Template objects can be built from it by
using `#with_locals`. Currently, this is just a convenience around
caching templates, but I hope it's a helpful concept that could have
more utility in the future.
