CONTRACTS: Move some functions from synthesizer_utils to contracts/utils

[qinheping]

Move some utils functions that will be used by GOTO-level loop contracts from synthesizer_utils to contracts/utils.

• Each commit message has a non-empty body, explaining why the change was made.
• Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Patch coverage: 81.81% and project coverage change: +0.08 🎉

Comparison is base (14fce73) 78.49% compared to head (0351b58) 78.58%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7782      +/-   ##
===========================================
+ Coverage    78.49%   78.58%   +0.08%     
===========================================
  Files         1693     1693              
  Lines       193381   193381              
===========================================
+ Hits        151796   151961     +165     
+ Misses       41585    41420     -165     

Impacted Files	Coverage Δ
src/goto-instrument/contracts/utils.h	100.00% <ø> (ø)
src/goto-synthesizer/synthesizer_utils.cpp	90.90% <ø> (+7.57%)	⬆️
src/goto-instrument/contracts/utils.cpp	92.07% <81.81%> (-2.28%)	⬇️

... and 11 files with indirect coverage changes

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