Contracts/DFCC: split conjunctions in loop invariants

[tautschnig]

Emitting one assertion (and assumption) per conjunct simplifies debugging when proving a loop invariant fails. It also appears to improve performance when using Bitwuzla, taking one example from more than 30 minutes down to 8 seconds. On the same example, performance using Z3 was not substantially different.

• 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/
• 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.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 78.35%. Comparing base (f68cf8c) to head (eb5466d).
Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../contracts/dynamic-frames/dfcc_instrument_loop.cpp	80.00%	4 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8458      +/-   ##
===========================================
- Coverage    78.37%   78.35%   -0.03%     
===========================================
  Files         1726     1726              
  Lines       188663   188733      +70     
  Branches     18265    18268       +3     
===========================================
+ Hits        147864   147874      +10     
- Misses       40799    40859      +60     

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