Create custom sequence to access child nodes

[5sw]

Also add a method to directly return the Document from DownASTRenderable.

[codecov]

Codecov Report

Merging #228 into master will increase coverage by 0.04%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master     #228      +/-   ##
==========================================
+ Coverage   68.63%   68.68%   +0.04%     
==========================================
  Files          74       75       +1     
  Lines        2047     2047              
==========================================
+ Hits         1405     1406       +1     
+ Misses        642      641       -1     

Impacted Files	Coverage Δ
Source/AST/Nodes/BaseNode.swift	100.00% <ø> (+9.52%)	⬆️
Tests/AST/NodeTests.swift	0.00% <0.00%> (ø)
Source/AST/Nodes/ChildSequence.swift	77.77% <77.77%> (ø)
Source/Renderers/DownASTRenderable.swift	86.66% <80.00%> (-3.34%)	⬇️
Source/AST/Nodes/Node.swift	100.00% <100.00%> (ø)
Source/AST/Visitors/Visitor.swift	92.30% <100.00%> (ø)
...rce/Renderers/DownAttributedStringRenderable.swift	50.00% <100.00%> (-4.55%)	⬇️
Tests/AST/VisitorTests.swift	98.60% <100.00%> (-0.01%)	⬇️

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[johnxnguyen]

Sorry for the delay in the review, I started this a few days ago but forgot to submit.

For me it is good, just want to check with @iwasrobbed-ks regarding the breaking change to DownASTRenderable.

[iwasrobbed-ks]

A couple of small tweaks, otherwise looks good! Thanks @5sw

[iwasrobbed-ks]

Thanks again @5sw 🎉