Fix Operations::Negation issues.

[hwo411]

• Fix inconsistency when #call calls []
• Add [] method to prevent errors when it's required, e.g. with And

I didn't find the specific issue, but this PR fixes usages of negation like

• Using negation inside of And or another operations when calling method [] (I used double negation for test)
• Calling not on schema in dry-validation(this probably needs also adding [] for schemas)
• Inconsistency between call and [], i.e. all call methods use call and [] methods use [] which is not the case for Negation

[hwo411]

@GustavoCaso @solnic @flash-gordon could you take a look please?

[solnic]

Thank you