Add new syntax to bind for-in loop indices

[dsainati1]

Closes #402

Description

This adds a new piece of syntax to for loops to the language to allow users to bind the index into the array being iterated over.

for x, y in array {
}

This introduces two variables x and y into the scope of the loop body; y is the element variable as in a normal for-in loop, while x is of type int, is initialized to 0 and is incremented by one at the end of each iteration of the loop, effectively tracking the index of the array being looped over.

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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
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[codecov-commenter]

Codecov Report

Merging #1216 (1f37d41) into master (28cd2f1) will increase coverage by 0.01%.
The diff coverage is 94.28%.

❗ Current head 1f37d41 differs from pull request most recent head ede9404. Consider uploading reports for the commit ede9404 to get more accurate results

@@            Coverage Diff             @@
##           master    #1216      +/-   ##
==========================================
+ Coverage   77.12%   77.14%   +0.01%     
==========================================
  Files         272      272              
  Lines       33911    33944      +33     
==========================================
+ Hits        26155    26186      +31     
- Misses       6693     6694       +1     
- Partials     1063     1064       +1     

Flag	Coverage Δ
unittests	77.14% <94.28%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
runtime/ast/statement.go	77.56% <ø> (ø)
runtime/sema/check_for.go	92.72% <85.71%> (-2.40%)	⬇️
runtime/interpreter/interpreter_statement.go	89.47% <100.00%> (+0.43%)	⬆️
runtime/parser2/statement.go	90.00% <100.00%> (+0.31%)	⬆️
runtime/common/address.go	92.00% <0.00%> (-0.31%)	⬇️

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[turbolent]

Nice work! 👍

BTW: We usually use a branch naming scheme like <name>/<issue-nunber>-<short-name> to avoid conflicts, e.g. for this PR/branch daniel/402-for-loop-index

[SupunS]

Is it safe to change the existing value, or should we assign a new value? I think changing the content of the existing value might also mutate the usages of previous indexes as well.

like for eg: what if a user stores the indexes in an array, and prints them later

[turbolent]

Ah, that's a good point. I had suggested this above to avoid allocation, but I think you are right

[dsainati1]

I added a test for this, and there are is indeed an error in this case. Nice catch

[turbolent]

@dsainati1 Nice, good idea to add a test! Feel free to revert to your original code, that worked and I think was safe 👍

[turbolent]

Nice work! 👍