Rewrite Patchers as a special case of multiple case classes mapping

[MateuszKubuszok]

Once #115 is implemented, patching could become just a special case of patch.into[Obj].withFallbackValue(obj).transform:

• extended with lookup for implicit Patcher[Obj, Patch]
• having a different unused field policy (vide #248)
• probably defining a few more/different Option rules

This rewrite would allow Patchers to use every feature that Transformers have just by adapting the PatchingContext into TransformationContext.

While its possible to implement many existing feature requests with the current Patcher implementation, such additions would quite possible duplicate the implementation for Transformers and be scheduled for deletion from the start.