Add iterator

[Sytten]

This is my first attempt at #318

Couple of points before reviewing:

• I decided to base the implementation on ArrayBuffer since Iterable and Iterator are not JS types and I didn't feel comfortable adding them to the Type enum
• Still unsure if I like the name of the trait IntoJsIter since it's not really an Into trait like we generally see in rust, but it's similar in spirit to IntoJsFunc
• The parameters of IntoJsIter are a bit strange with the position. I use the position in my util crate https://github.com/DelSkayn/rquickjs/pull/319/files#diff-653c6889323b6067b2be818f8ff00f785927ae3beaaeaba4a034feb5ea154d4eR133 since I do some magic with the This to access the parent class to avoid copying the values and my function is stateless so I need some indication on where we are. But we might want to remove it from the trait and move it to IterFn and IterFnMut

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 81.02981% with 70 lines in your changes are missing coverage. Please review.

Project coverage is 68.75%. Comparing base (304db5d) to head (8b80197).

Files	Patch %	Lines
core/src/value/iterator/iterable.rs	70.33%	35 Missing ⚠️
core/src/value/iterator.rs	88.69%	26 Missing ⚠️
core/src/value/iterator/into_iter.rs	57.14%	9 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #320      +/-   ##
==========================================
+ Coverage   68.31%   68.75%   +0.43%     
==========================================
  Files          83       86       +3     
  Lines       12237    12606     +369     
==========================================
+ Hits         8360     8667     +307     
- Misses       3877     3939      +62     

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[Sytten]

Gentle ping @DelSkayn, whenever you have time I just want to avoid it getting stale :)