add immutable-states detector

[0xalpharush]

close https://github.com/trailofbits/slither-private/issues/333

[montyly]

We are missing the candidates that are written in the constructor directly, ex:

contract A{
	uint a;
	constructor(uint b) {
		a = b;
	}
}

Maybe we should split this detector in two? As adding the logic for the constructor will increase the complexity, it would make sense to differentiate variables can that be determined at compile time (constant), and variables that can be determined at deployment (immutable) into two distinct detectors.

We can reuse the functions (_constant_initial_expression / _is_valid_type / ..) and mostly change the filtering logic

[montyly]

Why removing openzepellin?

Maybe we should iterate over compilation_unit.contracts_derived here, so that we only consider the most derived contracts, and we avoid FP on potential constant variable that are written in a derived contract