[compiler] reuse tags

[lifeart]

If template has case like this:

{{#if this.name}}
   {{this.name}}
   {{format this.name}}
{{/if}}

We create 3 tags for same property this.name.
We could avoid it and create one tag.

We could do it in compile-time with let.
Or we could replace this. with custom proxy and have caching on this level.
In addition, we could transform {{this.name}} to {{get this "name"}} and use something like cellFor

[lifeart]

we should not reuse tags if used inside control flow (if) - #149