Support `Conditional` ternary operator

[zao111222333]

As https://github.com/sybila/biodivine-lib-bdd/issues/52, this PR supports Conditional ternary operator. Those features have been implemented:

• BooleanExpression: Add field Cond(Box<BooleanExpression>, Box<BooleanExpression>, Box<BooleanExpression>). And update Display for BooleanExpression.
• parser: Implement parser for Cond, with priority: imp < cond < or.
• safe_eval_expression: Implement Cond's BDD with exsited function fn if_then_else(a: &Bdd, b: &Bdd, c: &Bdd) -> Bdd

There is one potential todo validation:

• Validation on BDD equivalence of A ? B : C and (A & B) | (!A & C)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.00000% with 6 lines in your changes are missing coverage. Please review.

Project coverage is 90.06%. Comparing base (da57ff9) to head (cd9e9c1).

Files	Patch %	Lines
src/boolean_expression/_impl_boolean_expression.rs	42.85%	4 Missing :warning:
src/boolean_expression/_impl_parser.rs	88.23%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #53 +/- ## ========================================== - Coverage 90.30% 90.06% -0.24% ========================================== Files 25 26 +1 Lines 1970 1993 +23 ========================================== + Hits 1779 1795 +16 - Misses 191 198 +7 ```

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[daemontus]

I'll just merge this as-is and fix the clippy warning on main, so that you don't have to wait for me to re-run the workflow :) Otherwise it looks good, thank you!

[daemontus]

Released as 0.5.13.