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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[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!