support serde serialize/deserialize for BooleanExpression & Bdd

[zao111222333]

This pull request mainly modifies two parts:

• Add serde dependency, controlled by serde feature. Referred to https://github.com/reem/rust-ordered-float/blob/master/Cargo.toml#L19
• Using serde's derive to conditionally implement Serialize & Deserialize for BooleanExpression & Bdd. Referred to https://stackoverflow.com/questions/42551113/is-it-possible-to-conditionally-enable-an-attribute-like-derive

Then we can use follow setting to turn on serde serialize/deserialize.

biodivine-lib-bdd = {version="0.5" , features = ["serde"]}

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.57%. Comparing base (249c905) to head (f8ad672).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #54      +/-   ##
==========================================
- Coverage   90.57%   90.57%   -0.01%     
==========================================
  Files          26       25       -1     
  Lines        2059     2058       -1     
==========================================
- Hits         1865     1864       -1     
  Misses        194      194              

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[daemontus]

Good idea. I'll add this to BddVariableSet (so that one can serialize the "context" of the BDD as well) and release it as a new minor version.

[daemontus]

This should not be online as version 0.5.17. Thank you for the update!