Date Implementation

[jcdickinson]

This Pull Request fixes/closes #252.

It changes the following:

• Add [Global] internal slot that contains nothing. This is used to differentiate between constructor calls and function calls in the Date constructor.
• Add Date according to spec.

[jcdickinson]

Please review these changes (first commit), the Date constructor can be called as a regular function and behaves differently. In order to do that, I need to determine if this is global (the spec calls this "thisTarget is undefined", but there is no thisTarget in Boa).

[codecov]

Codecov Report

Merging #596 into master will increase coverage by 0.84%.
The diff coverage is 83.25%.

@@            Coverage Diff             @@
##           master     #596      +/-   ##
==========================================
+ Coverage   71.67%   72.51%   +0.84%     
==========================================
  Files         177      179       +2     
  Lines       11647    12553     +906     
==========================================
+ Hits         8348     9103     +755     
- Misses       3299     3450     +151     

Impacted Files	Coverage Δ
boa/src/builtins/mod.rs	22.58% <0.00%> (-0.76%)	⬇️
boa/src/builtins/object/mod.rs	38.81% <0.00%> (-0.36%)	⬇️
boa/src/builtins/value/val_type.rs	24.00% <0.00%> (-1.00%)	⬇️
boa/src/builtins/value/mod.rs	65.49% <50.00%> (-0.44%)	⬇️
boa/src/builtins/date/mod.rs	69.75% <69.75%> (ø)
boa/src/builtins/value/operations.rs	31.36% <80.00%> (+0.48%)	⬆️
boa/src/builtins/date/tests.rs	99.30% <99.30%> (ø)
boa/src/builtins/object/tests.rs	100.00% <100.00%> (ø)
boa/src/realm.rs	72.22% <100.00%> (+1.63%)	⬆️
... and 2 more

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[jcdickinson]

Marking this as ready for review. This will likely need to be revisited when test262 works.

[Razican]

Looking good, check my comments :)