[exec Array] implement spread operator using iterator

[croraf]

This Pull Request fixes #793

It changes the following:

• Makes spread operator to use iterator as per spec https://tc39.es/ecma262/#sec-runtime-semantics-arrayaccumulation
• Breaks Map spreading, because Map does not implement iterator. That's why I opened Map.prototype[@@iterator]() method not implemented #810

[Razican]

Hey! thanks for this contribution. It seems it requires a re-base to fix one issue we were having in the test262 test suite :)

[codecov]

Codecov Report

Merging #811 into master will decrease coverage by 0.11%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #811      +/-   ##
==========================================
- Coverage   59.37%   59.26%   -0.12%     
==========================================
  Files         157      157              
  Lines       10036    10040       +4     
==========================================
- Hits         5959     5950       -9     
- Misses       4077     4090      +13     

Impacted Files	Coverage Δ
boa/src/context.rs	61.71% <ø> (-5.41%)	⬇️
boa/src/syntax/ast/node/array/mod.rs	74.07% <77.77%> (+0.16%)	⬆️

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[Razican]

Looks pretty good to me. Check if the mentioned changes make sense.

[Razican]

This is basically because we don't follow the spec with arrays. The thing is, arrays have a number of integer-indexed elements, and a nextIndex. this nextIndex will be the same as array.length, and it's defined here: https://tc39.es/ecma262/#sec-array-initializer-runtime-semantics-evaluation

But, due to our array being a vector (which causes performance issues), we don't have this defined. This is partially the reason behind ##816. This means that if we for example create a huge empty array (which should not allocate), say new Array(50000), we will have huge performance issues.

This also means that if we do something like let a = [5,6,,,6,7,,8];, we will have an undefined value in the non-defined slots, but they should instead be empty slots, and here, nextIndex would be 8 (same as length).

[croraf]

What do you suggest? Should we leave this comment and commented next_index, or modify the comment, or remove completly?

[Razican]

I would change the comment to a TODO, explaing this is waiting for a proper internal Array representation.

[croraf]

Please check the newly committed rewording.