CONTRACTS: make contracts.h methods public.

[remi-delmas-3000]

Modification needed in order to reuse some methods with the upcoming dynamic frames implementation.
No behaviour or performance impact.

• Each commit message has a non-empty body, explaining why the change was made.
• [N/A] Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• [N/A] The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• [N/A] 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).
• [N/A] 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

Merging #7084 (a4d33ee) into develop (9bbce17) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #7084   +/-   ##
========================================
  Coverage    77.85%   77.85%           
========================================
  Files         1574     1574           
  Lines       181235   181235           
========================================
  Hits        141099   141099           
  Misses       40136    40136           

Impacted Files	Coverage Δ
src/goto-instrument/contracts/contracts.h	100.00% <ø> (ø)
...c/solvers/smt2_incremental/convert_expr_to_smt.cpp	82.35% <ø> (ø)
..._incremental/smt2_incremental_decision_procedure.h	75.00% <ø> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	95.98% <100.00%> (ø)
...ncremental/smt2_incremental_decision_procedure.cpp	98.00% <100.00%> (ø)

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