Add an AST walking function

[turbolent]

Description

Add a function to walk the AST, similar to Go's function: https://golang.org/src/go/ast/walk.go

The implementation of Walk for the AST elements currently only visits child elements. A future version could also visit more information, such as identifiers, types, etc.

The current implementation is enough to determine the parent element of each element.

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• Targeted PR against master branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
• Added appropriate labels

[SupunS]

Nice addition to the visitors!

[codecov-commenter]

Codecov Report

Merging #939 (7aaf2da) into master (159015d) will decrease coverage by 0.14%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #939      +/-   ##
==========================================
- Coverage   74.20%   74.05%   -0.15%     
==========================================
  Files         267      269       +2     
  Lines       33001    33067      +66     
==========================================
  Hits        24487    24487              
- Misses       7386     7452      +66     
  Partials     1128     1128              

Flag	Coverage Δ
unittests	74.05% <0.00%> (-0.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
runtime/ast/block.go	87.50% <0.00%> (-7.96%)	⬇️
runtime/ast/composite.go	56.25% <0.00%> (-1.20%)	⬇️
runtime/ast/expression.go	66.74% <0.00%> (-3.33%)	⬇️
runtime/ast/function_declaration.go	63.26% <0.00%> (-2.70%)	⬇️
runtime/ast/import.go	73.33% <ø> (ø)
runtime/ast/inspect.go	0.00% <0.00%> (ø)
runtime/ast/interface.go	81.25% <0.00%> (-5.42%)	⬇️
runtime/ast/pragma.go	68.75% <0.00%> (-4.59%)	⬇️
runtime/ast/program.go	71.92% <0.00%> (-1.29%)	⬇️
runtime/ast/statement.go	72.89% <0.00%> (-8.87%)	⬇️
... and 6 more

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