Add support for ES iterators

[maxnordlund]

Summary

Proposed change:

This requires the Symbol.iterator well known symbol and Array.from to coerce them into vanilla arrays.

This basically supersedes #760, see there for more info.

Checklist

I've completed the checklist below to ensure I didn't forget anything. This makes reviewing this PR as easy as possible for the maintainers. And it gets this change released as soon as possible.

• Proposed change helps towards purpose of this project.
• Documentation is added / updated to describe proposed change.
• Tests are added / updated to cover proposed change.
• Changelog has an entry for proposed change (if user-facing fix or feature).

[fdintino]

Just wanted to give you a heads-up that there is a fairly big refactor / cleanup in progress and under discussion at PR #1053. I intend to merge that into master sometime next week probably. Your best bet would be to try to open this as a patch onto the develop branch.

I'm not sure about this approach when it comes to generators. Someone using nunjucks that wanted to convert their generators to arrays could use Array.from themselves, whereas true support would actually use Generator.prototype.next() and only lazily evaluate results during iteration. That being said, I don't see an issue with using Array.from for Map and Set.

[maxnordlund]

Ok, I've updated this PR to point to develop and your totally right about the generators, so I've removed that from the docs.

Talking about generators, you really want to use for of here, but that requires transpilation if it's going to work in all browsers.

[codecov-io]

Codecov Report

Merging #1058 into develop will increase coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1058      +/-   ##
===========================================
+ Coverage    89.41%   89.44%   +0.02%     
===========================================
  Files           22       22              
  Lines         2966     2974       +8     
===========================================
+ Hits          2652     2660       +8     
  Misses         314      314

Impacted Files	Coverage Δ
nunjucks/src/runtime.js	90.18% <100%> (+0.44%)	⬆️
nunjucks/src/compiler.js	95.5% <100%> (ø)	⬆️

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[fdintino]

It looks like you have a typo here (and below) for 'builtin'.

Would it also be possible to rewrite this to use this.skip? e.g.:

it('should work with the Set builtin', function() {
  /* global Set */
 if (typeof Set === 'undefined') {
    this.skip();
  } else {
    equal(/* ... */);
  }
});

[maxnordlund]

Done

[fdintino]

This is great. Thanks for your contribution!

[maxnordlund]

Always happy to help 😄

[ricordisamoa]

Did you mean #1058? 😅

[maxnordlund]

Whoops, but I also want to mention that PR and I didn't understand what the format was. Would you mind making a PR that corrects this, please?

[ricordisamoa]

#1061

[maxnordlund]

Thank you