Add support for marking redeclared methods as overrides

[kylesluder]

Under the current scheme (f6a99a4), redeclaring a method in an @interface block that belongs to a different namespace produces a declaration of a method in that namespace. The only way to tell the compiler "no, I really just want to advertise to clients that I implement this method" is to move the redeclaration into a category that belongs to the namespace of the declaration. Without that, the compiler would complain that the @implementation block is lacking a definition of a method with the same name as the redeclared method, but in a different namespace:

@namespace MyNS
@interface MyObject : NSObject
- (instancetype)init;

@implementation MyObject
@end
// ^ error: definition of -[MyNS init] in namespace 'MyNS' not found

@end

This is actually kind of fragile, since it requires updating the subclasses whenever the namespaces of their superclasses change. (This might actually be an instance of a larger issue.)

It might be preferable to take the heuristic described for @implementation blocks and make it opt-in within the @interface as well. This could be done with an override keyword:

@namespace MyNS
@interface MyObject : NSObject
- override (instancetype) init; // resolves to @selector(default, init)
@end

If the override is omitted, the compiler should probably warn about the ambiguity:

@interface MyNS
@interface MyObject: NSObject
- (instancetype) init;
// ^ warning: instance method -init declared in namespace 'default' by superclass 'NSObject'
@end

The intent to declare a method of the same name in a different namespace can be made explicit via another keyword. I can't think of a good one right now, but perhaps alongside would work (since the new method lives "alongside" the one in the other namespace):

@interface MyNS
@interface MyObject : NSObject
- alongside (instancetype) init;
@end