CHC solver: redesigned verbose output

[kroening]

The solver's output when using --verbose is cleaned up (and now less verbose).

• 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

Base: 77.99% // Head: 78.00% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (5d2bf4e) compared to base (9d51ea2).
Patch coverage: 87.21% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #7248   +/-   ##
========================================
  Coverage    77.99%   78.00%           
========================================
  Files         1619     1621    +2     
  Lines       187194   187272   +78     
========================================
+ Hits        146007   146085   +78     
  Misses       41187    41187           

Impacted Files	Coverage Δ
src/cprover/axioms.cpp	92.11% <ø> (+0.54%)	⬆️
src/cprover/console.cpp	29.06% <0.00%> (-2.19%)	⬇️
src/cprover/console.h	66.66% <ø> (ø)
src/cprover/counterexample_found.cpp	93.33% <ø> (+11.19%)	⬆️
src/cprover/state_encoding.cpp	80.57% <ø> (+0.22%)	⬆️
unit/util/expr_cast/expr_cast.cpp	100.00% <ø> (ø)
src/cprover/propagate.cpp	73.52% <44.44%> (-1.48%)	⬇️
src/cprover/solver.cpp	81.53% <48.00%> (-1.38%)	⬇️
src/cprover/solver_progress.cpp	60.00% <60.00%> (ø)
src/solvers/flattening/boolbv_byte_extract.cpp	69.73% <75.00%> (-0.40%)	⬇️
... and 29 more

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[tautschnig]

Why not use messaget::debug logging? A lot of code is now bound to wither away.

[kroening]

Why not use messaget::debug logging? A lot of code is now bound to wither away.

Contemplating it -- the key issue was that the full output was really very difficult to read.