CONTRACTS: functions to compute root objects

[remi-delmas-3000]

• Assigns clause targets can contain slices expressions which requires some adaptations on top of object_descriptor_exprt::root_object
• Assignments LHS can contain ternary operators, which requires computing a set of root object expressions instead of a single expression.

Extracted from #7541, will be tested when all new features are merged.

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[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (53b50bc) 78.51% compared to head (6b828d5) 78.51%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7636   +/-   ##
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  Coverage    78.51%   78.51%           
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  Files         1674     1674           
  Lines       191935   191935           
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  Hits        150704   150704           
  Misses       41231    41231           

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[tautschnig]

There are a bunch of recurring comments of mine here. For pointer-need-not-be-operand-0 you might want to make use of goto-programs/pointer_arithmetic.h to avoid having to go over all the operands.

[feliperodri]

#7629 also include these changes.

[remi-delmas-3000]

@tautschnig @feliperodri latest version, should be good to go (still needs approval from @tautschnig to merge)

[feliperodri]

@tautschnig if you have the time, could you do another round of review on this PR?

[tautschnig]

I believe this code can be simplified, see my comments.

[remi-delmas-3000]

@tautschnig all suggestions implemented, could you please take a last look ?