Improving variable search mechanism

[orchestr7]

What's done:

1. Adding mechanism for searching assignments of VARs
2. Moving all usages to new utilities of searching variables

[codecov]

Codecov Report

Merging #352 into master will increase coverage by 0.09%.
The diff coverage is 85.71%.

@@             Coverage Diff              @@
##             master     #352      +/-   ##
============================================
+ Coverage     83.00%   83.09%   +0.09%     
- Complexity     1263     1289      +26     
============================================
  Files            62       64       +2     
  Lines          3124     3147      +23     
  Branches        993      999       +6     
============================================
+ Hits           2593     2615      +22     
+ Misses          169      168       -1     
- Partials        362      364       +2     

Flag	Coverage Δ	Complexity Δ
#unittests	83.09% <85.71%> (+0.09%)	1289.00 <33.00> (+26.00)

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

Impacted Files	Coverage Δ	Complexity Δ
...tlin/org/cqfn/diktat/ruleset/utils/AstNodeUtils.kt	82.74% <ø> (-0.18%)	0.00 <0.00> (ø)
...at/ruleset/rules/identifiers/LocalVariablesRule.kt	91.66% <66.66%> (-0.40%)	24.00 <2.00> (-8.00)
...eset/utils/search/VariablesWithAssignmentSearch.kt	78.57% <78.57%> (ø)	6.00 <6.00> (?)
...qfn/diktat/ruleset/utils/search/VariablesSearch.kt	86.20% <86.20%> (ø)	18.00 <18.00> (?)
...cqfn/diktat/ruleset/rules/ImmutableValNoVarRule.kt	88.88% <86.66%> (+3.17%)	8.00 <0.00> (+3.00)
...in/org/cqfn/diktat/ruleset/rules/files/FileSize.kt	67.85% <100.00%> (+1.19%)	8.00 <0.00> (ø)
...fn/diktat/ruleset/rules/files/FileStructureRule.kt	57.89% <100.00%> (+0.75%)	19.00 <0.00> (ø)
.../ruleset/utils/search/VariablesWithUsagesSearch.kt	100.00% <100.00%> (ø)	7.00 <7.00> (?)
... and 1 more

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