Add PartialCached variant

[bep]

See

http://discuss.gohugo.io/t/how-to-make-hugo-faster/1597/6

{{ range $.Site.Pages.ByTitle }}
  {{ if and (in .Params.categories "Category") (in .Params.categories "sub-category") }}
    <li><a href="{{ .Permalink }}">{{ .Title }}</a>
        {{ if .Draft }} * {{ end }}</li>
  {{ end }}
{{ end }}

The above is an extract of a partial used on every page. It is an ineffective construct (slightly better in 0.15, I guess) and it gets really bad when page count goes up.

But it's static. And, although it is impossible to make a general assumption about how "cacheable" a given partial is (I think?), the user will know -- and should be able to say so.

So, for a site with 2000 pages, the above should only be needed to be processed once.

@tatsushid not sure how Ace include works in this area?

[tatsushid]

@bep, Ace parses a template file every time when the ParseSource and CompileResult or CompileResultWithTemplate are called. include is parsed when a base template file is read and is later (compile time) replaced by {{ template "template_name" pipeline }}. So using include is the same using template directly.

Ace caches parsed template files with these template names in a hash and if it finds a name in the hash, it bypasses parsing the template file, just returns a compile result which is a HTML file with Go template syntax.

But this cache mechanism is only implemented in ace.Load function. Hugo doesn't use it to merge all templates and use custom template function. So, instead of using Load, Hugo emulates the function without any cache mechanisms.

Anyway, Ace doesn't cache any rendering results. It just caches compiled result if Load is used. Executing template with values is needed.

[tatsushid]

Partial is used in both rendering with page specific values and rendering with common values to whole site. There is no way to know that at rendering time but only know a template author so introducing something "partial with cache" function is a reasonable way I think.

[LiaungYip]

I think the good people at Camunda are maintaining a version of Hugo with cached partials.

There was some discussion about submitting a PR to get that functionality into main.

See:

https://github.com/camunda-third-party/hugo/tree/camunda

particularly camunda-third-party@e15f8f5 :

feat(tpl): add cachedPartial template function
- allows to specify that a partial should be cached after first
  evaluation, useful for static complex partials often used partials

https://app.camunda.com/jira/browse/CAM-6005

https://app.camunda.com/jira/browse/CAM-6385

[bep]

Suggested API:

partialCached(name string, context interface{}, variant ...string) template.HTML

• Cache key constructed from name + (optional) variant
• Note that this will be called from multiple threads, so need a rw mutex

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