Reworked class design for data maskers:
Added interfaces IDictionaryMasker and IInstanceMasker. Classes
implement either or both interfaces depending on their capabilities.

Added AbstractInstBasedDictMasker for instance maskers that can be used
to mask whole dictionaries by performing their masking operation on each
of their elements. (It seems this applies to all instance maskers.)

Refactored all instance maskers to inherit from
AbstractInstBasedDictMasker, making them all automatically implement
IDictionaryMasker as well. Removed all of the now superfluous
dictionary-based maskers that used instance maskers.