Support array/dictionary subscript syntax via protocols

[andyarvanitis]

Currently, neither standard Objective-C nor Eero recognize subscript methods declared in protocols, e.g.:

@protocol TypedArray <NSObject, NSFastEnumeration>
-(instancetype) objectAtIndexedSubscript: (NSUInteger)index;
@end

NSNumber<TypedArray> * numberArray = ...
Number n = numberArray[0]; // compile error -- method to read element not found

It might be possible to add this ability to Eero.