Move ExprManager::isNAryKind to NodeManager. (#6107)

Move ExprManager::isNAryKind to NodeManager. (#6107)

This also renames metakind::getLowerBoundForKind and
metakind::getUpperBoundForKind for consistency.

Note that the NodeManager class needs to be reordered to comply to our
style guidelines. This PR does not reorder but introduces a public block
at the top (where the rest of the public interface of the class should
go eventually).