Feature gc module

[HalidOdat]

It changes the following:

• Put the gc stuff in the gc module. This cleans the root imports and adds some other gc stuff in the gc module

Instead of this

boa::{Value, Result, Trace, Finalize};

we can do this:

boa::{
	gc::{Trace, Finalize},
	Value, Result,
};

[codecov]

Codecov Report

Merging #694 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #694      +/-   ##
==========================================
- Coverage   71.70%   71.69%   -0.02%     
==========================================
  Files         199      199              
  Lines       13863    13865       +2     
==========================================
  Hits         9941     9941              
- Misses       3922     3924       +2     

Impacted Files	Coverage Δ
boa/examples/classes.rs	0.00% <ø> (ø)
boa/src/class.rs	0.00% <ø> (ø)
boa/src/lib.rs	81.81% <ø> (ø)
boa/src/context.rs	60.79% <0.00%> (-1.06%)	⬇️
boa/src/builtins/array/mod.rs	76.65% <0.00%> (ø)
boa/src/builtins/object/mod.rs	51.19% <0.00%> (+0.60%)	⬆️

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