[exec Map] Map.prototype.values and Map.prototype.keys implementation

[croraf]

This Pull Request closes #845

The number of changes is effectively small, but depends on #847 being merged first. Will do the merge of master after that.

[codecov]

Codecov Report

Merging #874 into master will increase coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #874      +/-   ##
==========================================
+ Coverage   59.17%   59.22%   +0.04%     
==========================================
  Files         165      165              
  Lines       10284    10290       +6     
==========================================
+ Hits         6086     6094       +8     
+ Misses       4198     4196       -2     

Impacted Files	Coverage Δ
boa/src/builtins/map/mod.rs	72.26% <66.66%> (-0.26%)	⬇️
boa/src/builtins/map/map_iterator.rs	71.42% <0.00%> (+7.14%)	⬆️

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