Multiplication encoding: cleanup, Dadda, data

[tautschnig]

Please review commit by commit: This PR does not change the encoding at all, but cleans up the multiplier code to have less defined-out code, fixes a TODO in Wallace tree reduction and adds data for Wallace trees, and finally adds a by-default-disabled implementation of Dadda's multiplier.

• 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.
• 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: 61 lines in your changes are missing coverage. Please review.

Comparison is base (25cb64d) 78.75% compared to head (1699176) 78.97%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7984      +/-   ##
===========================================
+ Coverage    78.75%   78.97%   +0.21%     
===========================================
  Files         1701     1701              
  Lines       196325   196386      +61     
===========================================
+ Hits        154625   155105     +480     
+ Misses       41700    41281     -419     

Files	Coverage Δ
src/solvers/flattening/bv_utils.h	86.66% <ø> (ø)
src/solvers/flattening/bv_utils.cpp	72.90% <16.43%> (-7.39%)	⬇️

... and 37 files with indirect coverage changes

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