Implement String.prototype.codePointAt

[devinus]

This Pull Request fixes/closes #751.

It changes the following:

• Adds String.prototype.codePointAt
• Adds a few more tests
• Fixes two Clippy linting suggestions

[codecov]

Codecov Report

Merging #935 (2731524) into master (7f5868b) will decrease coverage by 0.01%.
The diff coverage is 70.83%.

@@            Coverage Diff             @@
##           master     #935      +/-   ##
==========================================
- Coverage   59.21%   59.20%   -0.02%     
==========================================
  Files         166      166              
  Lines       10554    10563       +9     
==========================================
+ Hits         6250     6254       +4     
- Misses       4304     4309       +5     

Impacted Files	Coverage Δ
boa/src/builtins/string/mod.rs	61.47% <70.83%> (-0.34%)	⬇️

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[RageKnify]

Your changes look good, there was some nearby code that I was a bit confused by that I'd like another mantainer to check on.

[devinus]

Just as a heads-up, I tried testing corner cases around unpaired surrogates until I realized that required this to be completed first: #736

[Razican]

Thanks! It looks good to me :)