CONTRACTS: Allow NULL function pointers

[remi-delmas-3000]

Clauses requires_contract or ensures_contract
now accept NULL as third optional parameter
When provided, the meaning is: the function
pointer is either pointing to a function satisfying the given contract
or is NULL. When omitted, the function pointer cannot be NULL.

Drop support for these clauses in the old contract system.

Add user doc.

• 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

Base: 78.28% // Head: 78.29% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (d8a8e54) compared to base (75cd6a8).
Patch coverage: 78.16% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7319   +/-   ##
========================================
  Coverage    78.28%   78.29%           
========================================
  Files         1642     1642           
  Lines       190004   190008    +4     
========================================
+ Hits        148744   148761   +17     
+ Misses       41260    41247   -13     

Impacted Files	Coverage Δ
src/ansi-c/parser.y	81.09% <ø> (ø)
src/ansi-c/c_typecheck_code.cpp	78.88% <50.00%> (+1.51%)	⬆️
.../contracts/dynamic-frames/dfcc_wrapper_program.cpp	99.65% <97.61%> (+1.28%)	⬆️
src/ansi-c/c_expr.h	100.00% <100.00%> (ø)
src/goto-instrument/contracts/contracts.cpp	94.40% <100.00%> (-0.63%)	⬇️

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[remi-delmas-3000]

Replaced by two separate PRs: #7328 and #7327