VAR_TEMP support

[ghaith]

Added VAR_TEMP support to various POUs

VAR_TEMPs are not part of the interface, and will be allocated during the call

Closes #245

[codecov-commenter]

Codecov Report

Merging #336 (752a5ab) into master (c1c49bc) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #336      +/-   ##
==========================================
+ Coverage   95.38%   95.40%   +0.01%     
==========================================
  Files          52       52              
  Lines       14598    14618      +20     
==========================================
+ Hits        13924    13946      +22     
+ Misses        674      672       -2     

Impacted Files	Coverage Δ
src/codegen/generators/data_type_generator.rs	97.00% <100.00%> (+0.03%)	⬆️
src/codegen/generators/pou_generator.rs	96.45% <100.00%> (+0.20%)	⬆️
src/index.rs	92.45% <100.00%> (+0.04%)	⬆️
src/parser.rs	97.97% <100.00%> (+<0.01%)	⬆️
src/index/visitor.rs	96.40% <0.00%> (+0.25%)	⬆️
src/validation.rs	90.14% <0.00%> (+0.49%)	⬆️

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