Cache the mutable proxy of items w/in SSArrayDataSource
Cache the mutable proxy of items w/in SSArrayDataSource

This improves performance as the heavy -mutableArrayValueForKey: lookup
isn't needed on every access of SSArrayDataSource#items (which is
relatively frequent).

orta/SSDataSources,sgtsquiggs/SSDataSources,Anish-kumar-dev/SSDataSources,1debit/SSDataSources,splinesoft/SSDataSources