* Add NumericIndexValue
* MapConditionChecker: get rid of `F: Fn() -> bool` bound
To put this type into the upcoming `ConditionCheckerEnum`, it should be
nameable.
* filter_context: `Box<dyn ConditionChecker>` -> `OptimizedFilter`
Removes one level of dyn indirection, so faster checks.
* NullConditionChecker: merge IsEmpty/IsNull checkers into one
So, less variants in the upcoming `ConditionCheckerEnum`.