Allow creation of &dyn ClauseDatabase

[Dekker1]

To achieve this some of the (common) functionality previously contained
in the ClauseDatabase trait has been moved to a new
ClauseDabaseTools trait (named after Itertools). This allows us to
keep the same convenience methods.

[codecov]

Codecov Report

Attention: Patch coverage is 67.58242% with 59 lines in your changes missing coverage. Please review.

Project coverage is 75.11%. Comparing base (4245595) to head (0507024).

Files with missing lines	Patch %	Lines
crates/pindakaas/src/lib.rs	79.01%	17 Missing ⚠️
crates/pindakaas/src/bool_linear.rs	62.06%	11 Missing ⚠️
crates/pindakaas/src/solver/splr.rs	0.00%	10 Missing ⚠️
crates/pindakaas/src/propositional_logic.rs	25.00%	4 Missing and 2 partials ⚠️
crates/pindakaas/src/integer.rs	77.27%	4 Missing and 1 partial ⚠️
crates/pindakaas/src/solver/libloading.rs	0.00%	4 Missing ⚠️
crates/pindakaas/src/solver/propagation.rs	33.33%	2 Missing ⚠️
crates/pindakaas/src/sorted.rs	50.00%	2 Missing ⚠️
crates/pindakaas/src/helpers.rs	0.00%	0 Missing and 1 partial ⚠️
crates/pindakaas/src/trace.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #78      +/-   ##
===========================================
+ Coverage    74.86%   75.11%   +0.25%     
===========================================
  Files           20       20              
  Lines         7196     7098      -98     
  Branches      7196     7098      -98     
===========================================
- Hits          5387     5332      -55     
+ Misses        1686     1642      -44     
- Partials       123      124       +1     

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