Optimize BddVariableSet::new, 2x speedup

[zao111222333]

Hi!
I found there are some redundancy in BddVariableSet::new, this PR optimized that function in two ways:

• Using Iterator and collect, rather than Vec::push andHashMap::insert
• Directly build BddVariableSet.var_index_mapping, avoid BddVariableSetBuilder.var_names_set

I add test function to validate the correctness as well as performance, here are the results

---- _impl_bdd_variable_set::tests::bdd_new_performance stdout ----
New BddVariableSet::new runtime: 18.004839ms
Old BddVariableSet::new runtime: 35.596261ms

Hoping that looks good to you

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 90.81%. Comparing base (e805cd3) to head (bc41250).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/_impl_bdd_variable_set.rs	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #62      +/-   ##
==========================================
- Coverage   90.91%   90.81%   -0.11%     
==========================================
  Files          27       27              
  Lines        2158     2167       +9     
==========================================
+ Hits         1962     1968       +6     
- Misses        196      199       +3     

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[daemontus]

Thank you! This looks good. We never really thought to optimize the BddVariableSet constructor because it usually isn't the bottleneck, but this is certainly a useful change :)

[daemontus]

Should be published now as 0.5.23.

[zao111222333]

Thank you so much for your review!